Documentation/technical: describe multi-pack reverse indexes

Documentation/technical: describe multi-pack reverse indexes

As a prerequisite to implementing multi-pack bitmaps, motivate and
describe the format and ordering of the multi-pack reverse index.

The subsequent patch will implement reading this format, and the patch
after that will implement writing it while producing a multi-pack index.

Co-authored-by: Jeff King <redacted>
Signed-off-by: Jeff King <redacted>
Signed-off-by: Taylor Blau <redacted>
Signed-off-by: Junio C Hamano <redacted>