docs(contributing): add documenting guidelines from wiki (#19924)

Co-authored-by: Thomas Vigouroux <thomas.vigouroux@protonmail.com>

CONTRIBUTING.md

@@ -266,6 +266,41 @@ For managing includes in C files, use [include-what-you-use].
 
 See [#549][549] for more details.
 
+Documenting
+-----------
+
+Many parts of the `:help` documentation are autogenerated from C or Lua docstrings using the `./scripts/gen_vimdoc.py` script.
+You can filter the regeneration based on the target (api, lua, or lsp), or the file you changed, that need a doc refresh using `./scripts/gen_vimdoc.py -t <target>`.
+
+## Lua docstrings
+
+Lua documentation uses a subset of [EmmyLua] annotations. A rough outline of a function documentation is
+
+```lua
+--- {Brief}
+---
+--- {Long explanation}
+---
+---@param arg1 type {description}
+---@param arg2 type {description}
+{...}
+---
+---@return type {description}
+```
+
+If possible, always add type information (`table`, `string`, `number`, ...). Multiple valid types are separated by a bar (`string|table`). Indicate optional parameters via `type|nil`.
+
+If a function in your Lua module should not be documented (e.g. internal function or local function), you should set the doc comment to:
+
+```
+---@private
+```
+
+Mark functions that are deprecated as 
+```
+---@deprecated
+```
+
 Reviewing
 ---------
 
@@ -292,6 +327,7 @@ as context, use the `-W` argument as well.
 [Merge a Vim patch]: https://github.com/neovim/neovim/wiki/Merging-patches-from-upstream-Vim
 [complexity:low]: https://github.com/neovim/neovim/issues?q=is%3Aopen+is%3Aissue+label%3Acomplexity%3Alow
 [conventional_commits]: https://www.conventionalcommits.org
+[EmmyLua]: https://github.com/sumneko/lua-language-server/wiki/Annotations
 [gcc-warnings]: https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html
 [gh]: https://cli.github.com/
 [git-bisect]: http://git-scm.com/book/en/v2/Git-Tools-Debugging-with-Git