shallow.h: document '{commit,rollback}_shallow_file'

shallow.h: document '{commit,rollback}_shallow_file'

When 'commit_shallow_file()' and 'rollback_shallow_file()' were
introduced, they did not have a documenting comment, when they could
have benefited from one.

Add a brief note about what these functions do, and make a special note
that they reset stat-validity checks.

Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>