Explain generic Protocol[T1, T2, ...] shorthand in Mypy docs (#12047)

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.

docs/source/generics.rst

@@ -673,6 +673,10 @@ protocols mostly follow the normal rules for generic classes. Example:
    y: Box[int] = ...
    x = y  # Error -- Box is invariant
 
+Per :pep:`PEP 544: Generic protocols <544#generic-protocols>`, ``class
+ClassName(Protocol[T])`` is allowed as a shorthand for ``class
+ClassName(Protocol, Generic[T])``.
+
 The main difference between generic protocols and ordinary generic
 classes is that mypy checks that the declared variances of generic
 type variables in a protocol match how they are used in the protocol