bind: added protocol documentation to Objective-C generated file

Fixes golang/go#46956

Change-Id: Ifa1b8e7f2a0e46034f0eccc32249f69a064199e3
GitHub-Last-Rev: 175fa08b1696fca4c11b35b691e0df550ed66864
GitHub-Pull-Request: golang/mobile#67
Reviewed-on: https://go-review.googlesource.com/c/mobile/+/331429
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
Reviewed-by: Hajime Hoshi <hajimehoshi@gmail.com>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

bind/genobjc.go

@@ -894,6 +894,7 @@ func (g *ObjcGen) genInterfaceInterface(obj *types.TypeName, summary ifaceSummar
 }
 
 func (g *ObjcGen) genInterfaceH(obj *types.TypeName, t *types.Interface) {
+	doc := g.docs[obj.Name()]
 	summary := makeIfaceSummary(t)
 	if !summary.implementable {
 		g.genInterfaceInterface(obj, summary, false)
@@ -906,6 +907,7 @@ func (g *ObjcGen) genInterfaceH(obj *types.TypeName, t *types.Interface) {
 			continue
 		}
 		s := g.funcSummary(nil, m)
+		g.objcdoc(doc.Member(m.Name()))
 		g.Printf("- %s;\n", s.asMethod(g))
 	}
 	g.Printf("@end\n")

bind/testdata/doc.objc.h.golden

@@ -18,6 +18,9 @@
 @class DocI;
 
 @protocol DocI <NSObject>
+/**
+ * IM is a method.
+ */
 - (void)im;
 @end