pack-write: add explanation to promisor file content

pack-write: add explanation to promisor file content

In the entire codebase there is no explanation as to why the ".promisor"
files may contain the ref names (and their associated hashes) that were
fetched at the time the corresponding packfile was downloaded.

As explained in the log message of commit 5374a290 (fetch-pack: write
fetched refs to .promisor, 2019-10-14), where this loop originally came
from, these ref names (and associated hashes) are not used for anything
in the production, but are solely there to help debugging.

Explain this in a new comment.

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: LorenzoPegorari <lorenzo.pegorari2002@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>