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

src/backend/access/transam/xlog.c

@@ -3481,7 +3481,10 @@ XLogFileInit(XLogSegNo logsegno, bool *use_existent, bool use_lock)
 	if (fd < 0)
 		ereport(ERROR,
 				(errcode_for_file_access(),
-				 errmsg("could not open file \"%s\": %m", path)));
+				 errmsg("could not open file \"%s\": %m", path),
+				 (AmCheckpointerProcess() ?
+				  errhint("This is known to fail occasionally during archive recovery, where it is harmless.") :
+				  0)));
 
 	elog(DEBUG2, "done creating and filling new WAL file");
 

src/backend/storage/file/fd.c

@@ -834,7 +834,10 @@ durable_rename_excl(const char *oldfile, const char *newfile, int elevel)
 		ereport(elevel,
 				(errcode_for_file_access(),
 				 errmsg("could not link file \"%s\" to \"%s\": %m",
-						oldfile, newfile)));
+						oldfile, newfile),
+				 (AmCheckpointerProcess() ?
+				  errhint("This is known to fail occasionally during archive recovery, where it is harmless.") :
+				  0)));
 		return -1;
 	}
 	unlink(oldfile);
@@ -844,7 +847,10 @@ durable_rename_excl(const char *oldfile, const char *newfile, int elevel)
 		ereport(elevel,
 				(errcode_for_file_access(),
 				 errmsg("could not rename file \"%s\" to \"%s\": %m",
-						oldfile, newfile)));
+						oldfile, newfile),
+				 (AmCheckpointerProcess() ?
+				  errhint("This is known to fail occasionally during archive recovery, where it is harmless.") :
+				  0)));
 		return -1;
 	}
 #endif