[3.12] gh-107507: Replace 'The goals of Argument Clinic' with a summary (GH-107508) (#107516)

Summarise the goals of Argument Clinic in a single sentence.
Mention that Argument Clinic was introduced with PEP-436.
(cherry picked from commit abb71c6a8f73482c910ffdf050a86089a48e0e60)

Co-authored-by: Erlend E. Aasland <erlend@python.org>