chore(master): Improve empty reserved files docs (#747)

Explicitly mark EMPTY_RESERVED_FILES_PERIOD_MSECONDS as experimental
and add a detailed warning to clarify the potential risks of enabling
periodic reserved file cleanup.

The warning highlights possible disruption of active workloads and
advises administrators to use this option with caution.

Signed-off-by: Rolando Sánchez Ramos <rolysr@leil.io>

Author: rolysr

doc/sfsmaster.cfg.5.adoc

@@ -290,6 +290,23 @@ for TLS connections (there is no default value).
 Path to the trusted CA certificate which is used to authenticate
 the TLS connection (there is no default value).
 
+*EMPTY_RESERVED_FILES_PERIOD_MSECONDS (EXPERIMENTAL)*::
+Interval for periodic cleaning of reserved files, in milliseconds.
+If set to 0, the reserved files deletion is disabled. (Default: 0)
+
+[WARNING]
+====
+Administrator-only option. Enabling periodic deletion may remove master-side
+references to reserved files that are still held by other applications or
+client instances that have not released them yet. This can disrupt ongoing
+operations and lead to unexpected errors.
+
+Recommended approach: Do not enable automatic deletion unless you fully
+understand the impact on running workloads. Prefer manually freeing reserved
+files by locating each reference or process using them and stopping it before
+releasing the reservation.
+====
+
 == NOTES
 
 Chunks in master are tested in loop. Speed (or frequency) is regulated by two

src/data/sfsmaster.cfg.in

@@ -244,11 +244,6 @@
 ## (Default: 3600)
 # METADATA_DUMP_PERIOD_SECONDS = 3600
 
-## Interval for periodically cleaning of reserved files, in milliseconds. If set to 0, the reserved files
-## deletion is disabled.
-## (Default: 0)
-# EMPTY_RESERVED_FILES_PERIOD_MSECONDS = 0
-
 # deprecated:
 # CHUNKS_DEL_LIMIT - use CHUNKS_SOFT_DEL_LIMIT instead
 
@@ -355,3 +350,14 @@
 ## Example: /etc/saunafs/ssl/ca.crt
 ## (There is no default)
 # TLS_CA_CERT_FILE =
+
+## Interval for periodic cleaning of reserved files, in milliseconds.
+## If set to 0, the reserved files deletion is disabled.
+## (Default: 0)
+##
+## Note: Administrator-only option. Enabling periodic deletion may remove
+## master-side references to reserved files that are still held by other
+## applications or client instances that have not released them yet. This
+## can disrupt ongoing operations and lead to unexpected errors.
+##
+# EMPTY_RESERVED_FILES_PERIOD_MSECONDS = 0