[dev.regabi] cmd/compile: expand documentation for Func.Closure{Vars,Enter}

I keep getting these confused and having to look at how the code
actually uses them.

Change-Id: I86baf22b76e7dddada6830df0fac241092f716bf
Reviewed-on: https://go-review.googlesource.com/c/go/+/280993
Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>

src/cmd/compile/internal/ir/func.go

@@ -65,9 +65,22 @@ type Func struct {
 	// include closurevars until transformclosure runs.
 	Dcl []*Name
 
-	ClosureEnter Nodes   // list of ONAME nodes (or OADDR-of-ONAME nodes, for output parameters) of captured variables
-	ClosureType  Ntype   // closure representation type
-	ClosureVars  []*Name // closure params; each has closurevar set
+	ClosureType Ntype // closure representation type
+
+	// ClosureVars lists the free variables that are used within a
+	// function literal, but formally declared in an enclosing
+	// function. The variables in this slice are the closure function's
+	// own copy of the variables, which are used within its function
+	// body. They will also each have IsClosureVar set, and will have
+	// Byval set if they're captured by value.
+	ClosureVars []*Name
+
+	// ClosureEnter holds the expressions that the enclosing function
+	// will use to initialize the closure's free variables. These
+	// correspond one-to-one with the variables in ClosureVars, and will
+	// be either an ONAME node (if the variable is captured by value) or
+	// an OADDR-of-ONAME node (if not).
+	ClosureEnter Nodes
 
 	// Parents records the parent scope of each scope within a
 	// function. The root scope (0) has no parent, so the i'th