Add a few notes about intended usage of these backends

doc/man/man5/slapd-dnssrv.5

@@ -11,6 +11,8 @@ The DNSSRV backend to
 .BR slapd (8)
 serves up referrals based upon SRV resource records held in
 the Domain Name System.
+.LP
+This backend is experimental.
 .SH CONFIGURATION
 The DNSSRV backend has no backend nor database specific options.
 It is configured simply by "database dnssrv" followed a suffix

doc/man/man5/slapd-passwd.5

@@ -11,7 +11,7 @@ The PASSWD backend to
 .BR slapd (8)
 serves up the user account information listed in the system
 .BR passwd (5)
-file.
+file.  This backend is provided for demonstration purposes only.
 The DN of each entry is "uid=<username>,<suffix>".
 Note that non-base searches scan the the entire passwd file, and
 are best suited for hosts with small passwd files.

doc/man/man5/slapd-shell.5

@@ -13,6 +13,8 @@ executes external programs to implement operations, and is designed to
 make it easy to tie an existing database to the
 .B slapd
 front-end.
+.LP
+This backend is is primarily intended to be used in prototypes.
 .SH WARNING
 .B "This backend's calling conventions have changed since OpenLDAP 2.0."
 The abandon operation now gets a new "pid:" line.

doc/man/man5/slapd-sql.5

@@ -40,6 +40,8 @@ Also, it uses ODBC to connect to RDBMSes, and is highly configurable
 for SQL dialects RDBMSes may use, so it may be used for integration
 and distribution of data on different RDBMSes, OSes, hosts etc., in
 other words, in highly heterogeneous environment.
+.LP
+This backend is experimental.
 .SH CONFIGURATION
 These
 .B slapd.conf

doc/man/man5/slapd-tcl.5

@@ -14,6 +14,8 @@ interpreter into
 Any tcl database section of the configuration file
 .BR slapd.conf (5)
 must then specify what Tcl script to use.
+.LP
+This backend is experimental.
 .SH WARNING
 .B "This backend's calling conventions have changed since OpenLDAP 2.0."
 Previously, the 2nd argument to the procs was a message ID.