pgsql: Add HINT for restartpoint race with KeepFileRestoredFromArchive(

Add HINT for restartpoint race with KeepFileRestoredFromArchive().

The five commits ending at cc2c7d65fc27e877c9f407587b0b92d46cd6dd16
closed this race condition for v15+.  For v14 through v10, add a HINT to
discourage studying the cosmetic problem.

Reviewed by Kyotaro Horiguchi and David Steele.

Discussion: https://postgr.es/m/20220731061747.GA3692882@rfd.leadboat.com

Branch
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REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/8ad6c5dbbe5a234c55c6663020db297251756006

Modified Files
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src/backend/access/transam/xlog.c |  5 ++++-
src/backend/storage/file/fd.c     | 10 ++++++++--
2 files changed, 12 insertions(+), 3 deletions(-)