[vm/ffi] Cleanup old documentation references

`Pointer.load` and `Pointer.store` are now extension methods which are
only available for types that are sized.

`allocate` moved to `package:ffi` instead of `dart:ffi`.

Misc: gitignore another clangd cache location.

Change-Id: Ib01ba416f2efa18638a21dd4052d9a35eeab6f4f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/162188
Reviewed-by: Martin Kustermann <kustermann@google.com>
Reviewed-by: Tess Strickland <sstrickl@google.com>
Commit-Queue: Daco Harkes <dacoharkes@google.com>

.gitignore

@@ -61,6 +61,7 @@ compile_commands.json
 .gdb_history
 
 # Clangd files
+.cache/clangd
 .clangd
 
 # Built by chromebot and downloaded from Google Storage

sdk/lib/ffi/annotations.dart

@@ -32,6 +32,5 @@ class Unsized {
 /// This [NativeType] does not have predefined size.
 ///
 /// Unsized NativeTypes do not support [sizeOf] because their size is unknown.
-/// Consequently, [allocate], [Pointer.load], [Pointer.store], and
-/// [Pointer.elementAt] are not available.
+/// Consequently, [Pointer.elementAt] is not available.
 const unsized = const Unsized();