runtime: simplify description of FuncForPC behavior in case of inlining

The current description refers to the outermost "frame" which can be
misleading. A user reading it can think it means a stack frame.

Change-Id: Ie2c7cb4b4db8f41572df206478ce3b46a0245a5d
Reviewed-on: https://go-review.googlesource.com/47850
Reviewed-by: Austin Clements <[email protected]>

Author: ct1n

src/runtime/symtab.go

@@ -575,7 +575,7 @@ func moduledataverify1(datap *moduledata) {
 // 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.
+// the *Func describing the outermost function.
 func FuncForPC(pc uintptr) *Func {
 	return findfunc(pc)._Func()
 }