Documentation: add a note that shell integrations should not be just completions

[ambroisie]

A lot of PRs that I review which try to add shell integration options are only used for packages to install their shell completion, which should instead be done as part of packaging in nixpkgs, rather than in home manager.

A few examples from recent memory:

• jujutsu: fix completions commands #5016 and jujutsu: Remove superfluous shell completion #5037.
• uv: init module #6077.
• git-worktree-switcher: remove shell integrations #6440. EDIT: my mistake, this one is legitimate.
• others I forget.

We should explicitly document that a shell integration which only installs a program's completion is not necessary, and should instead be properly packaged in nixpkgs.

[boomshroom]

What should be done for packages that provide completions for shells other than bash, zsh, and fish? Home-manager supplies integrations for nushell for several packages, however nixpkgs only installs shell completions for the big three. That's not to mention other shells like elvish and powershell that some packages provide completions for, but are supported by neither home-manager nor nixpkgs.

[ambroisie]

In my opinion those should all be packaged in nixpkgs.