Document known lambda formatting deviations from Black (#21954)
Summary
--
Following #8179, we now format long lambda expressions a bit more like
Black, preferring to keep long parameter lists on a single line, but we
go one step further to break the body itself across multiple lines and
parenthesize it if it's still too long. This PR documents both the
stable deviation that breaks parameters across multiple lines, and the
new preview deviation that breaks the body instead.
I also fixed a couple of typos in the section immediately above my
addition.
Test Plan
--
I tested all of the snippets here against `main` for the preview
behavior, our playground for the stable behavior, and Black's playground
for their behavior
