Clarifying pg_rewind failure risks due to asynchronous file deletions

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/18/app-pgrewind.html
Description:

Reference Page: https://www.postgresql.org/docs/current/app-pgrewind.html

Proposed Enhancement: Include an explicit warning in the **"Description"**
or **"Warnings"** section regarding the requirement for the target data
directory to remain static during the rewind process.

Reasoning: While the documentation states that `pg_rewind` "will fail
immediately if it finds files it cannot write directly to," it does not
currently warn against asynchronous file deletions within the target
`$PGDATA` (or tablespace/WAL directories) while the utility is running.

In high-concurrency or automated environments, background processes—such as
the **WAL summarizer** cleanup, automated log rotations, or external
archiver tools (e.g., `pgbackrest` spooling)—may remove files that
`pg_rewind` has already indexed for synchronization. This results in a fatal
"No such file or directory" error during the final stages of the rewind,
often leaving the target directory in an unrecoverable state.

Suggested Phrasing:

"Warning: The target data directory must remain static during the rewind
operation. Any asynchronous file modifications or deletions—such as those
performed by automated log rotation, WAL summary cleanups, or external
backup tools—may cause `pg_rewind` to fail. If a file is removed after the
synchronization process has begun, the utility may error out, leaving the
target cluster in an inconsistent state that requires a new base backup."