Explain generic Protocol[T1, T2, ...] shorthand in Mypy docs

[posita]

As far as I can tell, the nugget exposed by this commit lives only in PEP 544. This
copies that nugget closer to where it is likely to be spotted by the intended audience.

PEPs may not be accessible to customers who reasonably expect relevant information to be
surfaced in featured documentation. Even if customers are aware of PEPs, they may not
think to look there, and don't likely consider them primary sources. It is reasonable to
assume that is the job of the docs, with PEPs capturing more esoteric nuances,
rationales and other then-relevant details of decision-making, etc. It's also reasonable
to expect that where further study may be helpful, links from relevant sections of
primary sources to secondary materials like PEPs should exist. This commit fills in both
gaps for the subject shorthand.

[posita]

make -C docs clean linkcheck returns no errors for generics. Screenshot of updated section built with make -C docs clean html:

FWIW, I'm not married to the wording. I'm open to truncating the example syntax as well. (The source material from PEP 544 is shorter.)

[JelleZijlstra]

Yes, I'd suggest removing all the ... and using only a single TypeVar. The ... makes it look like this is about variadics.

[posita]

I tried three variants, for fun:

[JelleZijlstra]

Thanks! I like the second one best: it makes it explicit that this applies to class declarations, but without the distracting ....

[posita]

Done.