Merge branch 'po/doc-branch-desc'

The branch descriptions that are set with "git branch --edit-description"
option were used in many places but they weren't clearly documented.

* po/doc-branch-desc:
doc: show usage of branch description

diff --cc Documentation/git-branch.txt
Simple merge

diff --cc Documentation/git-format-patch.txt
Simple merge

diff --cc Documentation/git-merge.txt
Simple merge