Clarify variance convention for Parameters #11360

Job	Run time
Test mypyc suite with 32-bit Python	13m 57s
Test suite with py38-ubuntu, mypyc-compiled	27m 25s
Test suite with py38-windows-64	33m 59s
Test suite with py39-ubuntu	24m 54s
Test suite with py310-ubuntu	32m 4s
Test suite with py311-ubuntu, mypyc-compiled	20m 22s
Test suite with py312-ubuntu, mypyc-compiled	26m 51s
mypyc runtime tests with py38-macos	9m 45s
mypyc runtime tests with py38-debug-build-ubuntu	7m 0s
Type check our own code (py38-ubuntu)	2m 17s
Type check our own code (py38-windows-64)	3m 9s
Formatting and code style with Black + ruff	1m 8s
	3h 22m 51s

Provide feedback