Merge branch 'sb/doc-worktree-prune-expire-improvement'

The help text and the documentation for the "--expire" option of
"git worktree [list|prune]" have been improved.

* sb/doc-worktree-prune-expire-improvement:
  worktree: clarify that --expire only affects missing worktrees

diff --cc Documentation/git-worktree.adoc
Simple merge

diff --cc builtin/worktree.c
Simple merge