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>

Documentation/git-branch.txt

@@ -22,13 +22,15 @@ SYNOPSIS
 DESCRIPTION
 -----------
 
-With no arguments, existing branches are listed and the current branch will
-be highlighted with an asterisk.  Option `-r` causes the remote-tracking
-branches to be listed, and option `-a` shows both. This list mode is also
-activated by the `--list` option (see below).
-<pattern> restricts the output to matching branches, the pattern is a shell
-wildcard (i.e., matched using fnmatch(3)).
-Multiple patterns may be given; if any of them matches, the branch is shown.
+If `--list` is given, or if there are no non-option arguments, existing
+branches are listed; the current branch will be highlighted with an
+asterisk.  Option `-r` causes the remote-tracking branches to be listed,
+and option `-a` shows both local and remote branches. If a `<pattern>`
+is given, it is used as a shell wildcard to restrict the output to
+matching branches. If multiple patterns are given, a branch is shown if
+it matches any of the patterns.  Note that when providing a
+`<pattern>`, you must use `--list`; otherwise the command is interpreted
+as branch creation.
 
 With `--contains`, shows only the branches that contain the named commit
 (in other words, the branches whose tip commits are descendants of the