MEDIUM: ssl: ssl-load-extra-del-ext work only with .crt

In order to be compatible with the "set ssl cert" command of the CLI,
this patch restrict the ssl-load-extra-del-ext to files with a ".crt"
extension in the configuration.

Related to issue #785.

Should be backported where 8e8581e ("MINOR: ssl: 'ssl-load-extra-del-ext'
removes the certificate extension") was backported.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 4d413be..5e38889 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -1377,9 +1377,11 @@
 ssl-load-extra-del-ext
   This setting allows to configure the way HAProxy does the lookup for the
   extra SSL files. By default HAProxy adds a new extension to the filename.
-  (ex: with "foobar.pem" load "foobar.pem.key"). With this option enabled,
+  (ex: with "foobar.crt" load "foobar.crt.key"). With this option enabled,
   HAProxy removes the extension before adding the new one (ex: with
-  "foobar.pem" load "foobar.key").
+  "foobar.crt" load "foobar.key").
+
+  Your crt file must have a ".crt" extension for this option to work.
 
   This option is not compatible with bundle extensions (.ecdsa, .rsa. .dsa)
   and won't try to remove them.