Document the mkdb_args argument for gnome.gtkdoc()

Include the newly added argument.

docs/markdown/Gnome-module.md

@@ -147,6 +147,7 @@ Compiles and installs gtkdoc documentation. Takes one positional arguments; The
 * `html_args` a list of arguments to pass to `gtkdoc-mkhtml`
 * `html_assets`: a list of assets for the HTML pages
 * `content_files`: a list of content files
+* `mkdb_args`: a list of arguments to pass to `gtkdoc-mkdb`
 
 This creates a `$module-doc` target that can be ran to build docs and normally these are only built on install.