doc/lisp: document arguments of buildASDFSystem

as `build-with-compile-into-pwd` for systems which create files during

compilation (such as cl-unicode).

The `build-asdf-system` function is documented with comments in

`nix-cl.nix`. Also, `packages.nix` is full of examples of how to use it.

The `build-asdf-system` function is documented

[here](#lisp-defining-packages-outside). Also, `packages.nix` is full of

examples of how to use it.

## Defining packages manually outside Nixpkgs {#lisp-defining-packages-outside}

function, which is similar to `build-asdf-system` from `packages.nix`, but is

part of the public API.

It takes the following arguments:

- `pname`: the package name

- `version`: the package version

- `src`: the package source

- `patches`: patches to apply to the source before build

- `nativeLibs`: native libraries used by CFFI and grovelling

- `javaLibs`: Java libraries for ABCL

- `lispLibs`: dependencies on other packages build with `buildASDFSystem`

- `systems`: list of systems to build

It can be used to define packages outside Nixpkgs, and, for example, add them

into the package scope with `withOverrides` which will be discussed later.

into the package scope with `withOverrides`.

### Including an external package in scope {#lisp-including-external-pkg-in-scope}