private/apigen: Add comment about panics

Add a comment in 2 methods about they panic as it's specified for the
`MustWriteDocs`.

Change-Id: I15d5fa420af37318d828bc633bed62988f8b207b

private/apigen/gogen.go

@@ -22,6 +22,7 @@ import (
 const DateFormat = "2006-01-02T15:04:05.999Z"
 
 // MustWriteGo writes generated Go code into a file.
+// If an error occurs, it panics.
 func (a *API) MustWriteGo(path string) {
 	generated, err := a.generateGo()
 	if err != nil {

private/apigen/tsgen.go

@@ -13,6 +13,7 @@ import (
 )
 
 // MustWriteTS writes generated TypeScript code into a file.
+// If an error occurs, it panics.
 func (a *API) MustWriteTS(path string) {
 	f := newTSGenFile(path, a)