task: auto-generate vimdoc

doc/langmapper.txt

@@ -1,4 +1,4 @@
-*langmapper.txt*       For Neovim >= 0.8.0      Last change: 2024 September 19
+*langmapper.txt*        For Neovim >= 0.8.0       Last change: 2024 October 22
 
 ==============================================================================
 Table of Contents                               *langmapper-table-of-contents*
@@ -125,6 +125,13 @@ Show default config ~
       ---@type string[] Names of layouts. If empty, will handle all configured layouts.
       use_layouts = {},
       ---@type table Fallback layouts
+      ---Custom description builder:
+      ---  old_desc - original description,
+      ---  method - 'translate' (map translated lhs) or 'feedkeys' (call `nvim_feedkeys` with original lhs)
+      ---  lhs - original left-hand side for translation
+      ---should return new description as a string. If error is occurs or non-string is returned, original builder with `LM ()` prefix will use
+      ---@type nil|function(old_desc, method, lhs): string
+      custom_desc = nil,
       layouts = {
         ---@type table Fallback layout item. Name of key is a name of language
         ru = {