Document negative refspec patterns

[michaelblyons]

• [x] I provide my work under the project license.
• [x] I grant such license of my work as is required for the purposes of future print editions to Ben Straub and Scott Chacon.

Changes

• Document negative refspecs (new in 2.29 from 2020)

If "segmenting" sounds too stilted, maybe "filtering" or another word could work. Just let me know!

Context

Fixes #1816