Improve pg_stat_recovery documentation

Hi,

While trying the new pg_stat_recovery view in v19, I found that it can return
no rows even while recovery is in progress. This happened because the user
querying the view lacked the pg_read_all_stats privilege.

That behavior seems fine to me, but it isn't documented. So I'd like to propose
the attached patch, which documents that behavior and also makes a few other
documentation improvements:

- Mention the view in high-availability.sgml as a way to monitor
recovery state and replay progress, alongside the existing recovery
information functions.
- Clarify that the view returns at most one row, not exactly one row,
and no rows to users who lack the pg_read_all_stats privilege.
- Correct the description of last_replayed_end_lsn to clarify that it
is the end LSN of the last replayed record plus one.
- Document that replay_end_tli equals last_replayed_tli when no WAL
record is currently being replayed.
- Clarify that current_chunk_start_time is NULL until streaming WAL
has been received.

Thought?

Regards,

--
Fujii Masao