xgbgen: process <doc> elements

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

nexgb/xgbgen/xml.go

@@ -92,6 +92,24 @@ type XMLErrorCopy struct {
 	Ref    string `xml:"ref,attr"`
 }
 
+type XMLDocField struct {
+	Name        string `xml:"name,attr"`
+	Description string `xml:",chardata"`
+}
+
+type XMLDocError struct {
+	Type        string `xml:"type,attr"`
+	Description string `xml:",chardata"`
+}
+
+type XMLDoc struct {
+	Brief       string         `xml:"brief"`
+	Description string         `xml:"description"`
+	Example     string         `xml:"example"`
+	Fields      []*XMLDocField `xml:"field"`
+	Errors      []*XMLDocError `xml:"error"`
+}
+
 type XMLStruct struct {
 	Name   string      `xml:"name,attr"`
 	Fields []*XMLField `xml:",any"`
@@ -108,10 +126,12 @@ type XMLRequest struct {
 	Combine bool        `xml:"combine-adjacent,attr"`
 	Fields  []*XMLField `xml:",any"`
 	Reply   *XMLReply   `xml:"reply"`
+	Doc     *XMLDoc     `xml:"doc"`
 }
 
 type XMLReply struct {
 	Fields []*XMLField `xml:",any"`
+	Doc    *XMLDoc     `xml:"doc"`
 }
 
 type XMLEvent struct {
@@ -119,6 +139,7 @@ type XMLEvent struct {
 	Number     int         `xml:"number,attr"`
 	NoSequence bool        `xml:"no-sequence-number,attr"`
 	Fields     []*XMLField `xml:",any"`
+	Doc        *XMLDoc     `xml:"doc"`
 }
 
 type XMLError struct {