DOC: ssl: Certificate hot update only works on fronted certificates

The CLI's "set ssl cert" command only works on frontend certificates but
the documentation did not specify this limitations yet.

This patch can be backported to all stable branches.
diff --git a/doc/management.txt b/doc/management.txt
index 4418328..7bca317 100644
--- a/doc/management.txt
+++ b/doc/management.txt
@@ -1948,6 +1948,8 @@
 set ssl cert <filename> <payload>
   This command is part of a transaction system, the "commit ssl cert" and
   "abort ssl cert" commands could be required.
+  This whole transaction system works on any certificate displayed by the
+  "show ssl cert" command, that is only frontend certificates for now.
   If there is no on-going transaction, it will duplicate the certificate
   <filename> in memory to a temporary transaction, then update this
   transaction with the PEM file in the payload. If a transaction exists with