[3.12] Docs: Improve multiprocessing.SharedMemory reference (#114093) (#114137)

(cherry picked from b1db6278cf6e60efdeca96244e5d73df4f070bea)

Align the multiprocessing shared memory docs with Diatáxis's
recommendations for references.

- use a parameter list for the SharedMemory.__init__() argument spec
- use the imperative mode
- use versionadded, not versionchanged, for added parameters
- reflow touched lines according to SemBr