Add description for new patterns supported in HBA and ident sample files

Support for regexps in database and role entries for pg_hba.conf has
been added in 8fea8683, and efb6f4a has extended support of pg-user in
pg_ident.conf, still both of them have missed a short description about
the new patterns supported in their respective sample files.

This commit closes the gap, by providing a short description of all the
new features supported for each entry type.

Reported-by: Pavel Luzanov
Reviewed-by: Jelte Fennema, Pavel Luzanov
Discussion: https://postgr.es/m/e495112d-8741-e651-64a2-ecb5728f1a56@postgrespro.ru

src/backend/libpq/pg_hba.conf.sample

@@ -31,14 +31,15 @@
 # - "hostnogssenc" is a TCP/IP socket that is not GSSAPI-encrypted
 #
 # DATABASE can be "all", "sameuser", "samerole", "replication", a
-# database name, or a comma-separated list thereof. The "all"
-# keyword does not match "replication". Access to replication
-# must be enabled in a separate record (see example below).
-#
-# USER can be "all", a user name, a group name prefixed with "+", or a
-# comma-separated list thereof.  In both the DATABASE and USER fields
-# you can also write a file name prefixed with "@" to include names
-# from a separate file.
+# database name, a regular expression (if it starts with a slash (/))
+# or a comma-separated list thereof.  The "all" keyword does not match
+# "replication".  Access to replication must be enabled in a separate
+# record (see example below).
+#
+# USER can be "all", a user name, a group name prefixed with "+", a
+# regular expression (if it starts with a slash (/)) or a comma-separated
+# list thereof.  In both the DATABASE and USER fields you can also write
+# a file name prefixed with "@" to include names from a separate file.
 #
 # ADDRESS specifies the set of hosts the record matches.  It can be a
 # host name, or it is made up of an IP address and a CIDR mask that is

src/backend/libpq/pg_ident.conf.sample

@@ -29,6 +29,10 @@
 # will be substituted for \1 (backslash-one) if present in
 # PG-USERNAME.
 #
+# PG-USERNAME can be "all", a user name, a group name prefixed with "+", or
+# a regular expression (if it starts with a slash (/)).  If it is a regular
+# expression, the substring matching with \1 has no effect.
+#
 # Multiple maps may be specified in this file and used by pg_hba.conf.
 #
 # No map names are defined in the default configuration.  If all