fetchers: document fetchDebianPatch

Most other fetchers return a directory rather than a single file.

## `fetchDebianPatch` {#fetchdebianpatch}

A wrapper around `fetchpatch`, which takes:

- `name` and `hash`: the patch's filename without the `.patch` suffix,

  and its hash after normalization by `fetchpatch` ;

- `pname`: the Debian source package's name ;

- `version`: the upstream version number ;

- `debianRevision`: the [Debian revision number] if applicable ;

- the `area` of the Debian archive: `main` (default), `contrib`, or `non-free`.

Here is an example of `fetchDebianPatch` in action:

```nix

{ lib

, fetchDebianPatch

, buildPythonPackage

}:

buildPythonPackage rec {

  pname = "pysimplesoap";

  version = "1.16.2";

  src = ...;

  patches = [

    (fetchDebianPatch {

      inherit pname version;

      debianRevision = "5";

      name = "Add-quotes-to-SOAPAction-header-in-SoapClient";

      hash = "sha256-xA8Wnrpr31H8wy3zHSNfezFNjUJt1HbSXn3qUMzeKc0=";

    })

  ];

  ...

}

```

Patches are fetched from `sources.debian.org`, and so must come from a

package version that was uploaded to the Debian archive.  Packages may

be removed from there once that specific version isn't in any suite

anymore (stable, testing, unstable, etc.), so maintainers should use

`copy-tarballs.pl` to archive the patch if it needs to be available in

the very long-term.

[Debian revision number]: https://www.debian.org/doc/debian-policy/ch-controlfields.html#version

## `fetchsvn` {#fetchsvn}

Used with Subversion. Expects `url` to a Subversion directory, `rev`, and `hash`.