pgsql: Improve comments in online checksums code

Improve comments in online checksums code

Spelling fixes and rewording outdated information.

Author: Tomas Vondra <tomas@vondra.me>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: /messages/by-id/f1281cf3-89a3-4936-9bc5-2a5a6291229f@vondra.me

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/cd857dec0e0ae63dabf548d51fddb27c45561086

Modified Files
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src/backend/access/transam/xlog.c | 5 ++---
src/backend/postmaster/auxprocess.c | 2 +-
src/backend/postmaster/datachecksum_state.c | 34 ++++++++++++-----------------
src/backend/utils/init/postinit.c | 2 +-
4 files changed, 18 insertions(+), 25 deletions(-)