runtime: document FuncForPC behavior with inlining

Change-Id: I1c02aa4f7131ae984fda66b32e8a993c0a40b8f4
Reviewed-on: https://go-review.googlesource.com/47690Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/symtab.go

@@ -573,6 +573,9 @@ func moduledataverify1(datap *moduledata) {
 
 // FuncForPC returns a *Func describing the function that contains the
 // given program counter address, or else nil.
+//
+// If pc represents multiple functions because of inlining, it returns
+// the *Func describing the outermost frame at pc.
 func FuncForPC(pc uintptr) *Func {
 	return findfunc(pc)._Func()
 }