Bug 1706325 - Minor tweaks to the VIM docs. r=sylvestre

Mostly mentioning that other completion engines also work, and how to
set jsm files to be treated like other JS files (and thus ESLint etc
would work on them).

Differential Revision: https://phabricator.services.mozilla.com/D112729

docs/contributing/editor.rst

@@ -30,11 +30,13 @@ AutoCompletion
 ~~~~~~~~~~~~~~
 
 There's C++ and Rust auto-completion support for VIM via
-`YouCompleteMe <https://github.com/ycm-core/YouCompleteMe/>`__.
+`YouCompleteMe <https://github.com/ycm-core/YouCompleteMe/>`__. As long as that
+is installed and you have run :code:`./mach build` or :code:`./mach configure`,
+it should work out of the box. Configuration for this lives in
+:code:`.ycm_extra_conf` at the root of the repo.
 
-As long as that is installed and you have run :code:`./mach build` or
-:code:`./mach configure`, it should work out of the box. Configuration for this lives
-in :code:`.ycm_extra_conf` at the root of the repo.
+If you don't like YouCompleteMe, other solutions also work, but they'll require
+you to create a :code:`compile_commands.json` file (see below for instructions).
 
 Rust auto-completion should work both with the default completer (RLS, as of
 this writing), or with `rust-analyzer <https://rust-analyzer.github.io/manual.html#youcompleteme>`__.
@@ -45,6 +47,13 @@ ESLint
 The easiest way to integrate ESLint with VIM is using the `Syntastic plugin
 <https://github.com/vim-syntastic/syntastic>`__.
 
+In order for VIM to detect jsm files as JS you might want something like this
+in your :code:`.vimrc`:
+
+.. code::
+
+    autocmd BufRead,BufNewFile *.jsm set filetype=javascript
+
 :code:`mach eslint --setup` installs a specific ESLint version and some ESLint
 plugins into the repositories' :code:`node_modules`.