pgsql: Document restrictions regarding incremental backups and standbys

Document restrictions regarding incremental backups and standbys.

If you try to take an incremental backup on a standby and there hasn't
been much system activity, it might fail. Document why this happens.
Also add a hint to the error message you get, to make it more likely
that users will understand what has gone wrong.

Laurenz Albe and Robert Haas

Discussion: https://postgr.es/m/5468641ad821dad7aa3b2d65bf843146443a1b68.camel@cybertec.at

Branch
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REL_17_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/de8b098ce567526e4da7d6e51cfe0fb132123ad7

Modified Files
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doc/src/sgml/backup.sgml                    | 11 +++++++++++
src/backend/backup/basebackup_incremental.c |  3 ++-
2 files changed, 13 insertions(+), 1 deletion(-)