go/ast: improve documentation for channel directions

Fixes #22815.

Change-Id: Iaa54fa311bf2c0549a6316a0d51cc92425f5ba94
Reviewed-on: https://go-review.googlesource.com/80855Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/go/ast/ast.go

@@ -356,8 +356,8 @@ type (
 	}
 )
 
-// The direction of a channel type is indicated by one
-// of the following constants.
+// The direction of a channel type is indicated by a bit
+// mask including one or both of the following constants.
 //
 type ChanDir int