xgbgen: process <doc> elements

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

nexgb/xgbgen/request_reply.go

@@ -6,6 +6,22 @@ import (
 	"unicode"
 )
 
+// Doc contains any documentation, if present. Example C code is excluded.
+type Doc struct {
+	Brief       string            // short description
+	Description string            // long description
+	Fields      map[string]string // from field name to description
+	Errors      map[string]string // from error type to description
+}
+
+// DescribeField is an accessor that supports nil receivers.
+func (d *Doc) DescribeField(name string) string {
+	if d == nil {
+		return ""
+	}
+	return d.Fields[name]
+}
+
 // Request represents all XML 'request' nodes.
 // If the request doesn't have a reply, Reply is nil.
 type Request struct {
@@ -15,6 +31,7 @@ type Request struct {
 	Combine bool    // Not currently used.
 	Fields  []Field // All fields in the request.
 	Reply   *Reply  // A reply, if one exists for this request.
+	Doc     Doc     // Documentation.
 }
 
 type Requests []*Request
@@ -126,6 +143,7 @@ func (r *Request) Size(c *Context) Size {
 // Reply encapsulates the fields associated with a 'reply' element.
 type Reply struct {
 	Fields []Field
+	Doc    Doc
 }
 
 // Size gets the number of bytes in this request's reply.