Merge pull request #3457 from teojgo/doc/db_lock

[doc] Document the requirement of file lock support for enabling results storage

Author: vkarak

docs/manpage.rst

@@ -1348,6 +1348,13 @@ Since the report file information is now kept in the results database, there is
 You can disable the report generation by turning off the :attr:`~config.general.generate_file_reports` configuration parameter.
 The file report of any session can be retrieved from the database with the :option:`--describe-stored-sessions` option.
 
+.. warning::
+
+    ReFrame uses file locking to coordinate storing of run session data in the database file.
+    Enabling the database storage on filesystems that do not support locking (e.g some networked filesystems) might lead to hangs at the end of a run session.
+    For this reason, you must make sure that the database file is located on a filesystem that supports locking.
+    You can set the database location through the :attr:`~config.storage.sqlite_db_file` configuration setting or the :envvar:`RFM_SQLITE_DB_FILE` environment variable.
+
 
 .. _querying-past-results: