[PATCH] doc: Add multiplier explanation for pg_resetwal -c option

Hi,

I noticed that the pg_resetwal documentation for the --commit-timestamp-ids
(-c) option was missing a multiplier explanation, unlike similar options
such as --oldest-transaction-id (-u). There's even a TODO comment in the
source asking about this.

This patch adds:
- Multiplier value (21824 / 0x5540) for calculating transaction IDs from
pg_commit_ts file names
- Concrete example showing file 0007 → 0x25480
- Comment documenting the multiplier formula

The multiplier is calculated as: SLRU_PAGES_PER_SEGMENT * BLCKSZ /
SizeOfCommitTimestampEntry = 32 * 8192 / 12 = 21824

This follows the same documentation pattern used for the -u option.

Patch attached.

Thanks,
Aviral Asthana