Add more general summary to vacuumlazy.c

Add more general summary to vacuumlazy.c

Add more comments at the top of vacuumlazy.c on heap relation vacuuming
implementation.

Previously vacuumlazy.c only had details related to dead TID storage.
This commit adds a more general summary to help future developers
understand the heap relation vacuum design and implementation at a high
level.

Reviewed-by: Alena Rybakina, Robert Haas, Andres Freund, Bilal Yavuz
Discussion: https://postgr.es/m/flat/CAAKRu_ZF_KCzZuOrPrOqjGVe8iRVWEAJSpzMgRQs%3D5-v84cXUg%40mail.gmail.com