Document support for self-referential extras

[ofek]

What's the problem this feature will solve?

This (coupled with maybe a new explicit test) will provide a guarantee to users that the following use case is supported:

[project]
name = "pkg"

[project.optional-dependencies]
all = ["pkg[a]", "pkg[b]"]
a = ["..."]
b = ["..."]

Describe the solution you'd like

Add a note in docs + a new test that explicitly checks this

Alternative Solutions

N/A

Additional context

@ichard26 found that this has been supported since 21.2 (tested by @henryiii) #10393 (comment)

Code of Conduct

• I agree to follow the PSF Code of Conduct.

[sbidoul]

I marked this as awaiting PR. I'd say a having adding a test is important if we document it.