[`flake8-pyi`] Improve motivation for `custom-type-var-return-type` (`PYI019`) #21835

Job	Run time
cargo fuzz	3m 37s
cargo clippy	1m 45s
cargo fmt	15s
benchmarks	9m 9s
cargo test (linux)	3m 28s
cargo test (wasm)	1m 7s
cargo test (windows)	5m 28s
cargo udeps	2m 6s
Determine changes	10s
mkdocs	2m 1s
pre-commit	27s
python package	1m 13s
test scripts	52s
formatter instabilities and black similarity	0s
test ruff-lsp	15s
ecosystem	7m 32s
	39m 25s

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