git-worktree.txt: make start of new sentence more obvious

When reading the rendered description of `add`, it's easy to trip over
and miss the end of one sentence and the start of the next, making it
seem as if they are part of the same statement, separated only by a
dash:

    ... specific files such as HEAD, index, etc. - may also be
    specified as <commit-ish>; it is synonymous with...

This can be particularly confusing since the thoughts expressed by the
two sentences are unrelated. Reduce the likelihood of confusion by
making it obvious that the two sentences are distinct.

Signed-off-by: Eric Sunshine <[email protected]>
Reviewed-by: Taylor Blau <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: sunshineco

Documentation/git-worktree.txt

@@ -48,8 +48,8 @@ add <path> [<commit-ish>]::
 
 Create `<path>` and checkout `<commit-ish>` into it. The new working directory
 is linked to the current repository, sharing everything except working
-directory specific files such as `HEAD`, `index`, etc. `-` may also be
-specified as `<commit-ish>`; it is synonymous with `@{-1}`.
+directory specific files such as `HEAD`, `index`, etc. As a convenience,
+`<commit-ish>` may be a bare "`-`", which is synonymous with `@{-1}`.
 +
 If `<commit-ish>` is a branch name (call it `<branch>`) and is not found,
 and neither `-b` nor `-B` nor `--detach` are used, but there does