docs/neovim: add plugin license notes

`PLUGIN.passthru.initLua` for neovim plugins. For instance, the `unicode-vim` plugin

needs the path towards a unicode database so we expose the following snippet `vim.g.Unicode_data_directory="${self.unicode-vim}/autoload/unicode"` under `vimPlugins.unicode-vim.passthru.initLua`.

### Plugin license overrides {#neovim-plugin-license-overrides}

Generated Vim and Neovim plugins get their `meta.license` from GitHub license metadata when possible.

Some upstream repositories do not expose a license file that GitHub can detect, or only mention the license in a README.

In those cases, add a manual `meta.license` override in [overrides.nix](https://github.com/NixOS/nixpkgs/blob/master/pkgs/applications/editors/vim/plugins/overrides.nix).

For example, if upstream documents that a plugin uses the Vim license but GitHub does not detect it:

```nix

{

  foo-nvim = super.foo-nvim.overrideAttrs (old: {

    meta = old.meta // {

      # README says this plugin is distributed under the Vim license.

      license = lib.licenses.vim;

    };

  });

}

```

## LuaRocks based plugins {#neovim-luarocks-based-plugins}

In order to automatically handle plugin dependencies, several Neovim plugins

    "index.html#neovim-plugin-required-snippet",

    "index.html#vim-plugin-required-snippet"

  ],

  "neovim-plugin-license-overrides": [

    "index.html#neovim-plugin-license-overrides"

  ],

  "updating-plugins-in-nixpkgs": [

    "index.html#updating-plugins-in-nixpkgs"

  ],