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.go

@@ -168,7 +168,13 @@ func (f *ExprField) Write(c *Context, prefix string) {
 
 // Value field
 func (f *ValueField) Define(c *Context) {
+	if f.MaskComment != "" {
+		c.PutComment(f.MaskComment)
+	}
 	c.Putln("%s %s", f.MaskName, f.SrcType())
+	if f.ListComment != "" {
+		c.PutComment(f.ListComment)
+	}
 	c.Putln("%s []uint32", f.ListName)
 }
 
@@ -197,6 +203,9 @@ func (f *ValueField) Write(c *Context, prefix string) {
 
 // Switch field
 func (f *SwitchField) Define(c *Context) {
+	if f.Comment != "" {
+		c.PutComment(f.Comment)
+	}
 	c.Putln("%s []uint32", f.Name)
 }