reflect: match MakeMapWithSize docs about initial capacity with spec

Following the spec clarification in CL 40393, copy that text
to reflect docs to state that the initial capacity of MakeMapWithSize
is a hint/approximate.

Fixes #19903

Change-Id: I6b3315b8183cafaa61fbb2839a4e42b76fd71544
Reviewed-on: https://go-review.googlesource.com/46270Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/reflect/value.go

@@ -2082,12 +2082,13 @@ func MakeMap(typ Type) Value {
 	return MakeMapWithSize(typ, 0)
 }
 
-// MakeMapWithSize creates a new map with the specified type and initial capacity.
-func MakeMapWithSize(typ Type, cap int) Value {
+// MakeMapWithSize creates a new map with the specified type
+// and initial space for approximately n elements.
+func MakeMapWithSize(typ Type, n int) Value {
 	if typ.Kind() != Map {
 		panic("reflect.MakeMapWithSize of non-map type")
 	}
-	m := makemap(typ.(*rtype), cap)
+	m := makemap(typ.(*rtype), n)
 	return Value{typ.common(), m, flag(Map)}
 }