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_request_reply.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"fmt"
+	"sort"
 	"strings"
 )
 
@@ -12,6 +13,33 @@ func (r *Request) Define(c *Context) {
 	c.Putln("*xgb.Cookie")
 	c.Putln("}")
 	c.Putln("")
+
+	if r.Doc.Description != "" {
+		c.PutComment(r.Doc.Description)
+		c.Putln("//")
+	}
+
+	allErrors := make([]string, 0, len(r.Doc.Errors))
+	for kind := range r.Doc.Errors {
+		allErrors = append(allErrors, kind)
+	}
+	sort.Strings(allErrors)
+
+	undocErrors := make([]string, 0)
+	for _, kind := range allErrors {
+		if desc := r.Doc.Errors[kind]; desc == "" {
+			undocErrors = append(undocErrors, kind)
+		} else {
+			c.PutComment(fmt.Sprintf("May return a %s error if %s%s", kind,
+				strings.ToLower(desc[:1]), desc[1:]))
+			c.Putln("//")
+		}
+	}
+	if len(undocErrors) > 0 {
+		c.Putln("// May return %s errors.", strings.Join(undocErrors, ", "))
+		c.Putln("//")
+	}
+
 	if r.Reply != nil {
 		c.Putln("// %s sends a checked request.", r.SrcName())
 		c.Putln("// If an error occurs, it will be returned with the reply "+