Merge branch 'mh/doc-remote-helpers'

* mh/doc-remote-helpers: git-remote-helpers.txt: clarify options & ref list attributes git-remote-helpers.txt: clarify command <-> capability correspondences git-remote-helpers.txt: rearrange description of capabilities git-remote-helpers.txt: minor grammar fix git-remote-helpers.txt: document missing capabilities git-remote-helpers.txt: document invocation before input format