This tries to clarify the fact that the branches must be remote and the syntax
for specifying them as globs.
Cc @yuja, https://github.com/martinvonz/jj/pull/2625#discussion_r1423379351
Here is the result (excerpt):
```
$ jj branch track --help
Start tracking given remote branches
A tracking remote branch will be imported as a local branch of the same name. Changes to it
will propagate to the existing local branch on future pulls.
Usage: jj branch track [OPTIONS] <BRANCH@REMOTE>...
Arguments:
<BRANCH@REMOTE>...
Remote branches to track
By default, the specified name matches exactly. Use `glob:` prefix to select
branches by wildcard pattern. For details, see
https://github.com/martinvonz/jj/blob/main/docs/revsets.md#string-patterns.
Examples: branch@remote, glob:main@*, glob:jjfan-*@upstream
```