docs: clarify git-branch --list behavior

It was not clear from the "description" section of
git-branch(1) that using a <pattern> meant that you _had_ to
use the --list option. Let's clarify that, and while we're
at it, reword some clunky and ambiguous sentences.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
branch: let branch filters imply --list

Currently, a branch filter like `--contains`, `--merged`, or
`--no-merged` is ignored when we are not in listing mode.
For example:

  git branch --contains=foo bar

will create the branch "bar" from the current HEAD, ignoring
the `--contains` argument entirely. This is not very
helpful. There are two reasonable behaviors for git here:

  1. Flag an error; the arguments do not make sense.

  2. Implicitly go into `--list` mode

This patch chooses the latter, as it is more convenient, and
there should not be any ambiguity with attempting to create
a branch; using `--contains` and not wanting to list is
nonsensical.

That leaves the case where an explicit modification option
like `-d` is given.  We already catch the case where
`--list` is given alongside `-d` and flag an error. With
this patch, we will also catch the use of `--contains` and
other filter options alongside `-d`.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>