patch 9.1.0538: not possible to assign priority when defining a sign

Problem:  not possible to assign priority when defining a sign
          (Mathias Fußenegger)
Solution: Add the priority argument for the :sign-define ex command and
          the sign_define() function (LemonBoy)

Use the specified value instead of the default one (SIGN_DEF_PRIO) when
no priority is explicitly specified in sign_place or :sign place.

fixes: #8334
closes: #15124

Signed-off-by: LemonBoy <thatlemon@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/version9.txt b/runtime/doc/version9.txt
index d70c3b1..92792b9 100644
--- a/runtime/doc/version9.txt
+++ b/runtime/doc/version9.txt
@@ -41576,6 +41576,7 @@
 - 'completeopt' is now a |global-local| option.
 - 'nrformat' accepts the new "blank" suboption, to determine a signed or
   unsigned number based on whitespace in front of a minus sign.
+- allow to specify a priority when defining a new sign |:sign-define|
 
 							*added-9.2*
 Added ~