cmd/compile: improve fncall docs

Comment changes only.

Change-Id: I3f9c1c38ae6b4989f02b62fff09265e4bcb934f7
Reviewed-on: https://go-review.googlesource.com/114519
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/compile/internal/gc/walk.go

@@ -1836,10 +1836,7 @@ func ascompatee(op Op, nl, nr []*Node, init *Nodes) []*Node {
 	return nn
 }
 
-// l is an lv and rt is the type of an rv
-// return 1 if this implies a function call
-// evaluating the lv or a function call
-// in the conversion of the types
+// fncall reports whether assigning an rvalue of type rt to an lvalue l might involve a function call.
 func fncall(l *Node, rt *types.Type) bool {
 	if l.HasCall() || l.Op == OINDEXMAP {
 		return true
@@ -1847,6 +1844,7 @@ func fncall(l *Node, rt *types.Type) bool {
 	if eqtype(l.Type, rt) {
 		return false
 	}
+	// There might be a conversion required, which might involve a runtime call.
 	return true
 }
 
@@ -1865,9 +1863,8 @@ func ascompatet(nl Nodes, nr *types.Type) []*Node {
 		}
 		r := nr.Field(i)
 
-		// any lv that causes a fn call must be
-		// deferred until all the return arguments
-		// have been pulled from the output arguments
+		// Any assignment to an lvalue that might cause a function call must be
+		// deferred until all the returned values have been read.
 		if fncall(l, r.Type) {
 			tmp := temp(r.Type)
 			tmp = typecheck(tmp, Erv)