(config.php.sample) Clarify the purpose of `session_keepalive` parameter #13272

Job	Run time
changes	10s
integration-sqlite (capabilities_features, 8.1, main)	3m 25s
integration-sqlite (collaboration_features, 8.1, main)	4m 37s
integration-sqlite (comments_features, 8.1, main)	3m 35s
integration-sqlite (dav_features, 8.1, main)	9m 41s
integration-sqlite (features, 8.1, main)	12m 40s
integration-sqlite (federation_features, 8.1, main)	7m 0s
integration-sqlite (--tags ~@large files_features, 8.1, main)	17m 1s
integration-sqlite (filesdrop_features, 8.1, main)	2m 48s
integration-sqlite (openldap_features, 8.1, main)	3m 15s
integration-sqlite (openldap_numerical_features, 8.1, main)	1m 9s
integration-sqlite (ldap_features, 8.1, main)	3m 10s
integration-sqlite (remoteapi_features, 8.1, main)	2m 31s
integration-sqlite (setup_features, 8.1, main)	2m 54s
integration-sqlite (sharees_features, 8.1, main)	5m 4s
integration-sqlite (sharing_features, 8.1, main)	26m 31s
integration-sqlite (videoverification_features, 8.1, main)	7m 41s
integration-sqlite-summary	5s
	1h 53m 17s

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