vimPlugins.nvim-treesitter: fix update.py and update documentation (#321535)

Nix expressions for Vim plugins are stored in [pkgs/applications/editors/vim/plugins](https://github.com/NixOS/nixpkgs/tree/master/pkgs/applications/editors/vim/plugins). For the vast majority of plugins, Nix expressions are automatically generated by running [`nix-shell -p vimPluginsUpdater --run vim-plugins-updater`](https://github.com/NixOS/nixpkgs/blob/master/pkgs/applications/editors/vim/plugins/updater.nix). This creates a [generated.nix](https://github.com/NixOS/nixpkgs/blob/master/pkgs/applications/editors/vim/plugins/generated.nix) file based on the plugins listed in [vim-plugin-names](https://github.com/NixOS/nixpkgs/blob/master/pkgs/applications/editors/vim/plugins/vim-plugin-names).

After running the updater, if nvim-treesitter received an update, also run [`nvim-treesitter/update.py`](https://github.com/NixOS/nixpkgs/blob/master/pkgs/applications/editors/vim/plugins/update.py) to update the tree sitter grammars for `nvim-treesitter`.

When the vim updater detects an nvim-treesitter update, it also runs [`nvim-treesitter/update.py $(nix-build -A vimPlugins.nvim-treesitter)`](https://github.com/NixOS/nixpkgs/blob/master/pkgs/applications/editors/vim/plugins/update.py) to update the tree sitter grammars for `nvim-treesitter`.

Some plugins require overrides in order to function properly. Overrides are placed in [overrides.nix](https://github.com/NixOS/nixpkgs/blob/master/pkgs/applications/editors/vim/plugins/overrides.nix). Overrides are most often required when a plugin requires some dependencies, or extra steps are required during the build process. For example `deoplete-fish` requires both `deoplete-nvim` and `vim-fish`, and so the following override was added:

if __name__ == "__main__":

    generated = update_grammars(sys.args[1])

    generated = update_grammars(sys.argv[1])

    open(join(os.path.dirname(__file__), "generated.nix"), "w").write(generated)