Add documentation for --watch-file

This PR adds documentation for the `--watch-file` option (that was missing). The behaviour for this option changed in mozilla/web-ext#2125.

Author: Dexterp37

src/content/documentation/develop/web-ext-command-reference.md

@@ -340,6 +340,14 @@ web-ext run --start-url www.mozilla.com --start-url developer.mozilla.org
 
 Environment variable: `$WEB_EXT_START_URL`
 
+#### `--watch-file`
+
+A list of files that should be watched for changes. This is useful if you want web-ext to explicitly watch for changes to specific files, without watching the extension directory tree, e.g. the output of the build from a module bundler.
+
+```shell
+web-ext run --watch-file dist/background.js dist/content-script.js
+```
+
 #### `--watch-ignored`
 
 A list of paths and globs patterns that should not be watched for changes. This is useful if you want to explicitly prevent web-ext from watching part of the extension directory tree, e.g. the node_modules folder.