nscd: Improve nscd.conf comments.

This change adds a warning to nscd.conf about running multiple caching
services together and that it may lead to unexpected behaviours. Also we
add a note that enabling the 'shared' option will cause cache hit rates
to be misreported (a side effect of the implementation).

v2
- Rewrite comment to avoid implementation details.

ChangeLog

@@ -1,3 +1,7 @@
+2019-08-19  Carlos O'Donell  <carlos@redhat.com>
+
+	* nscd/nscd.conf: Add warning and comment about shared option.
+
 2019-08-19  Carlos O'Donell  <carlos@redhat.com>
 
 	* nss/nsswitch.conf: Expand comments, and simplify defaults.

nscd/nscd.conf

@@ -3,6 +3,9 @@
 #
 # An example Name Service Cache config file.  This file is needed by nscd.
 #
+# WARNING: Running nscd with a secondary caching service like sssd may lead to
+#          unexpected behaviour, especially with how long entries are cached.
+#
 # Legal entries are:
 #
 #	logfile			<file>
@@ -23,6 +26,9 @@
 #	check-files		<service> <yes|no>
 #	persistent		<service> <yes|no>
 #	shared			<service> <yes|no>
+#	NOTE: Setting 'shared' to a value of 'yes' will accelerate the lookup,
+#	      but those lookups will not be counted as cache hits
+#	      i.e. 'nscd -g' may show '0%'.
 #	max-db-size		<service> <number bytes>
 #	auto-propagate		<service> <yes|no>
 #