no error if sbin/scch not found

If checkpoint/blcr is configured. only log that scch is not found
using debug() rather than info(). Add documentation about the file.

doc/man/man5/slurm.conf.5

@@ -204,7 +204,10 @@ Supported values presently include:
 for AIX systems only
 .TP
 \fBcheckpoint/blcr\fR
-Berkeley Lab Checkpoint Restart (BLCR)
+Berkeley Lab Checkpoint Restart (BLCR).
+NOTE: If a file is found at sbin/scch (relative to the SLURM installation
+location), it will be executed upon completion of the checkpoint. This can
+be a script used for managing the checkpoint files.
 .TP
 \fBcheckpoint/none\fR
 no checkpoint support (default)

src/plugins/checkpoint/blcr/checkpoint_blcr.c

@@ -698,7 +698,10 @@ static int _on_ckpt_complete(uint32_t group_id, uint32_t user_id,
 	pid_t cpid;
 
 	if (access(scch_path, R_OK | X_OK) < 0) {
-		info("Access denied for %s: %m", scch_path);
+		if (errno == ENOENT)
+			debug("checkpoint/blcr: file %s not found", scch_path);
+		else
+			info("Access denied for %s: %m", scch_path);
 		return SLURM_ERROR;
 	}