doc/go1.18: add documentation for changes to go/types

The number of involved CLs is too large (hundreds) so
no CLs are mentioned in (html) comments.

Updates #47694

Change-Id: I655d800a1e56a71e9d70a190f1c42c17baf6861e
Reviewed-on: https://go-review.googlesource.com/c/go/+/369099
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

doc/go1.18.html

@@ -451,6 +451,93 @@ Do not send CLs removing the interior tags from such phrases.
   </dd>
 </dl>
 
+<dl id="go/types"><dt><a href="/pkg/go/types/">go/types</a></dt>
+  <dd>
+    <p><!-- https://golang.org/issue/47916 -->
+      Per the proposal
+      <a href="https://go.googlesource.com/proposal/+/master/design/47916-parameterized-go-types.md">
+        Additions to go/types to support type parameters
+      </a>
+      the following additions are made to the <a href="/pkg/go/types"><code>go/types</code></a> package:
+    </p>
+    <ul>
+      <li>
+        The new type
+        <a href="/pkg/go/types/#TypeParam"><code>TypeParam</code></a>, factory function
+        <a href="/pkg/go/types/#NewTypeParam"><code>NewTypeParam</code></a>,
+        and associated methods were added to represent a type parameter.
+      </li>
+      <li>
+        The new type
+        <a href="/pkg/go/types/#TypeParamList"><code>TypeParamList</code></a> holds a list of
+        type parameters.
+      </li>
+      <li>
+        The new type
+        <a href="/pkg/go/types/#TypeList"><code>TypeList</code></a> and factory function
+        <a href="/pkg/go/types/#NewTypeList"><code>NewTypeList</code></a> facilitate storing
+        a list of types.
+      </li>
+      <li>
+        The new factory function
+        <a href="/pkg/go/types/#NewSignatureType"><code>NewSignatureType</code></a> allocates a
+        <a href="/pkg/go/types/#Signature"><code>Signature</code></a> with
+        (receiver or function) type parameters.
+        To access those type parameters, the <code>Signature</code> type has two new methods
+        <a href="/pkg/go/types/#Signature.RecvTypeParams"><code>Signature.RecvTypeParams</code></a> and
+        <a href="/pkg/go/types/#Signature.TypeParams"><code>Signature.TypeParams</code></a>.
+      </li>
+      <li>
+        <a href="/pkg/go/types/#Named"><code>Named</code></a> types have four new methods:
+        <a href="/pkg/go/types/#Named.Origin"><code>Named.Origin</code></a> to get the original
+        parameterized types of instantiated types,
+        <a href="/pkg/go/types/#Named.TypeArgs"><code>Named.TypeArgs</code></a> and
+        <a href="/pkg/go/types/#Named.TypeParams"><code>Named.TypeParams</code></a> to get the
+        type arguments or type parameters of an instantiated or parameterized type, and
+        <a href="/pkg/go/types/#Named.TypeParams"><code>Named.SetTypeParams</code></a> to set the
+        type parameters (for instance, when importing a named type where allocation of the named
+        type and setting of type parameters cannot be done both at once due to possible cycles).
+      </li>
+      <li>
+        The <a href="/pkg/go/types/#Interface"><code>Interface</code></a> type has four new methods:
+        <a href="/pkg/go/types/#Interface.IsComparable"><code>Interface.IsComparable</code></a> and
+        <a href="/pkg/go/types/#Interface.IsMethodSet"><code>Interface.IsMethodSet</code></a> to
+        query properties of the type set defined by the interface, and
+        <a href="/pkg/go/types/#Interface.MarkImplicit"><code>Interface.MarkImplicit</code></a> and
+        <a href="/pkg/go/types/#Interface.IsImplicit"><code>Interface.IsImplicit</code></a> to set
+        and test whether the interface is an implicit interface around a type constraint literal.
+      </li>
+      <li>
+        The new types
+        <a href="/pkg/go/types/#Union"><code>Union</code></a> and
+        <a href="/pkg/go/types/#Term"><code>Term</code></a>, factory functions
+        <a href="/pkg/go/types/#NewUnion"><code>NewUnion</code></a> and
+        <a href="/pkg/go/types/#NewTerm"><code>NewTerm</code></a>, and associated
+        methods were added to represent type sets in interfaces.
+      </li>
+      <li>
+        The new function
+        <a href="/pkg/go/types/#Instantiate"><code>Instantiate</code></a>
+        instantiates a parameterized type.
+      </li>
+      <li>
+        The new <a href="/pkg/go/types/#Info.Instances"><code>Info.Instances</code></a>
+        map records function and type instantiations through the new
+        <a href="/pkg/go/types/#Instance"><code>Instance</code></a> type.
+      </li>
+      <li><!-- CL 342671 -->
+        The new type <a href="/pkg/go/types/#ArgumentError"><code>ArgumentError</code></a>
+        and associated methods were added to represent an error related to a type argument.
+      </li>
+      <li><!-- CL 353089 -->
+        The new type <a href="/pkg/go/types/#Context"><code>Context</code></a> and factory function
+        <a href="/pkg/go/types/#NewContext"><code>NewContext</code></a>
+        were added to facilitate sharing of identical type instances across type-checked packages.
+      </li>
+    </ul>
+  </dd>
+</dl>
+
 <dl id="image/draw"><dt><a href="/pkg/image/draw/">image/draw</a></dt>
   <dd>
     <p><!-- CL 340049 -->