docs(clue): mention that desc field can be function

Related to #1526

Author: echasnovski

doc/mini-clue.txt

@@ -536,8 +536,8 @@ A clue table is a table with the following fields:
 - <keys> `(string)` - key combination for which clue will be shown.
   "Human-readable" key names as in |key-notation| (like "<Leader>", "<Space>",
   "<Tab>", etc.) are allowed.
-- <desc> `(string|nil)` - optional key combination description which will
-  be shown in clue window.
+- <desc> `(string|function|nil)` - optional key combination description which is
+  shown in clue window. If function, should return string description.
 - <postkeys> `(string|nil)` - optional postkeys which will be executed
   automatically after `keys`. Allows creation of submodes
   (see |MiniClue-examples-submodes|).

lua/mini/clue.lua

@@ -532,8 +532,8 @@ end
 --- - <keys> `(string)` - key combination for which clue will be shown.
 ---   "Human-readable" key names as in |key-notation| (like "<Leader>", "<Space>",
 ---   "<Tab>", etc.) are allowed.
---- - <desc> `(string|nil)` - optional key combination description which will
----   be shown in clue window.
+--- - <desc> `(string|function|nil)` - optional key combination description which is
+---   shown in clue window. If function, should return string description.
 --- - <postkeys> `(string|nil)` - optional postkeys which will be executed
 ---   automatically after `keys`. Allows creation of submodes
 ---   (see |MiniClue-examples-submodes|).