cmd/compile: add doc comment to the parseFiles function

Change-Id: Ifa14557ba834865602c207297ccf0c63e93feb4c
Reviewed-on: https://go-review.googlesource.com/117695
Run-TryBot: Yury Smolsky <yury@smolsky.by>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

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

@@ -19,6 +19,10 @@ import (
 	"cmd/internal/src"
 )
 
+// parseFiles concurrently parses files into *syntax.File structures.
+// Each declaration in every *syntax.File is converted to a syntax tree
+// and its root represented by *Node is appended to xtop.
+// Returns the total count of parsed lines.
 func parseFiles(filenames []string) uint {
 	var noders []*noder
 	// Limit the number of simultaneously open files.