runtime: update comment for Callers
runtime: update comment for Callers

Attempt to clear up confusion about how to turn
the PCs reported by Callers into the file and line
number people actually want.

Fixes issue 7690.

LGTM=r, chris.cs.guy
R=r, chris.cs.guy
CC=golang-codereviews
https://codereview.appspot.com/163550043

bryanxu/go-zh,bryanxu/go-zh,bryanxu/go-zh,webfd/go-zh,d0f/go-zh,d0f/go-zh,webfd/go-zh,d0f/go-zh,bryanxu/go-zh,bryanxu/go-zh,bryanxu/go-zh,d0f/go-zh,webfd/go-zh,webfd/go-zh,d0f/go-zh,d0f/go-zh,webfd/go-zh,bryanxu/go-zh,d0f/go-zh,webfd/go-zh,d0f/go-zh,webfd/go-zh,bryanxu/go-zh,webfd/go-zh