xgbgen: process <doc> elements

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

nexgb/xgbgen/field.go

@@ -105,6 +105,7 @@ type SingleField struct {
 	srcName string
 	xmlName string
 	Type    Type
+	Comment string
 }
 
 func (f *SingleField) Initialize(p *Protocol) {
@@ -255,10 +256,12 @@ func (f *ExprField) Initialize(p *Protocol) {
 // integers. The mask specifies which kinds of values are in the list.
 // (i.e., See ConfigureWindow, CreateWindow, ChangeWindowAttributes, etc.)
 type ValueField struct {
-	Parent   interface{}
-	MaskType Type
-	MaskName string
-	ListName string
+	Parent      interface{}
+	MaskType    Type
+	MaskName    string
+	ListName    string
+	MaskComment string
+	ListComment string
 }
 
 func (f *ValueField) SrcName() string {
@@ -324,6 +327,7 @@ type SwitchField struct {
 	MaskName string
 	Expr     Expression
 	Bitcases []*Bitcase
+	Comment  string
 }
 
 func (f *SwitchField) SrcName() string {