nixos/doc: remove reference to long removed option

# Customising Packages {#sec-customising-packages}

Some packages in Nixpkgs have options to enable or disable optional

functionality or change other aspects of the package. For instance, the

Firefox wrapper package (which provides Firefox with a set of plugins

such as the Adobe Flash player) has an option to enable the Google Talk

plugin. It can be set in `configuration.nix` as follows:

`nixpkgs.config.firefox.enableGoogleTalkPlugin = true;`

functionality or change other aspects of the package.

::: {.warning}

Unfortunately, Nixpkgs currently lacks a way to query available

:::

::: {.note}

Alternatively, many packages come with extensions one might add.

For example, many packages come with extensions one might add.

Examples include:

- [`passExtensions.pass-otp`](https://search.nixos.org/packages/query=passExtensions.pass-otp)

- [`python310Packages.requests`](https://search.nixos.org/packages/query=python310Packages.requests)