debug/gosym: adjust go12* method comments
debug/gosym: adjust go12* method comments

These methods are for use with Go 1.2 _and later_ pcln tables.
Make that clearer.

Change-Id: Iee06e0828fd5895639b654363b6d91bf9151d224
Reviewed-on: https://go-review.googlesource.com/c/go/+/352950
Trust: Josh Bleecher Snyder <cdc248cc4d32e4fbfe04e45228163bcecd48a0b9@gmail.com>
Reviewed-by: Cherry Mui <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>

golang/go,golang/go,CAFxX/go,cloudflare/go,cloudflare/go,CAFxX/go,nawawi/go,CAFxX/go,cloudflare/go,cloudflare/go,golang/go,cloudflare/go,CAFxX/go,nawawi/go,CAFxX/go,CAFxX/go,CAFxX/go,nawawi/go,cloudflare/go,golang/go,nawawi/go,nawawi/go,golang/go,golang/go,cloudflare/go,golang/go,golang/go,cloudflare/go,nawawi/go,nawawi/go,nawawi/go,CAFxX/go