xgbgen: process <doc> elements

Most of XCB documentation now ends up in Go sources,
although the end result is of mixed quality.

nexgb/xgbgen/go_single_field.go

@@ -6,6 +6,9 @@ import (
 )
 
 func (f *SingleField) Define(c *Context) {
+	if f.Comment != "" {
+		c.PutComment(f.Comment)
+	}
 	c.Putln("%s %s", f.SrcName(), f.Type.SrcName())
 }