debug/elf: add comments explaining applyRelocations for amd64/arm64
debug/elf: add comments explaining applyRelocations for amd64/arm64

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/155190043

tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go