pgsql: 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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/3edc67d337c2e498dad1cd200e460f7c63e512e6

Modified Files
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src/backend/access/heap/vacuumlazy.c | 54 ++++++++++++++++++++++++++++++++++++
1 file changed, 54 insertions(+)