Fix inconsistencies with code and beautify xlog structures description and fin hash_xlog.h

Hi hackers!

While doing some xlog-related task at my job I noticed inconsistency
between [0] & [1]. hash_xlog.h xl_hash_squeeze_page structure
description mismatches with actual code.
PFA fixing issue.

I also allowed myself to standardise description of
xl_hash_move_page_contents and xl_hash_init_bitmap_page, so
xl_hash_delete, xl_hash_init_bitmap_page and
xl_hash_move_page_contents describes their zeroth backup block akin to
each other.


[0]
https://git.postgresql.org/cgit/postgresql.git/tree/src/include/access/hash_xlog.h?id=567d27e8e2b752743626eb259ba75ecdc936eaf3#n147
[1]
https://git.postgresql.org/cgit/postgresql.git/tree/src/backend/access/hash/hash_xlog.c?id=567d27e8e2b752743626eb259ba75ecdc936eaf3#n830

-- 
Best regards,
Kirill Reshke