docs/manual: using a branch name as FOO_VERSION does not work

docs/manual: using a branch name as FOO_VERSION does not work

For various reasons, we've always suggested users to avoid using a
branch as version string for their packages, because it does not work
as a they would expect:

  - it is not reproducible, because the branch may change between two
    builds that are done at different times;

  - it does not even follow the branch, as Buildroot anyway generates
    a local tarball, which it will reuse on subsequent builds.
    Furthermore, since we fetch and not pull, any existing local branch
    is not updated.

Yet, until recently, using a branch name would just work (with the
above limitations): the git tree was cloned, the branch checked out,
and the tarball created.

But with the advent of the git caching, using a branch name does not
work anymore. Indeed, we now do a git-fetch, and that does not create
a local master branch. So we can't check out master, because it does
not exist locally. And for other branches, as noticed above, the local
branch does not get udpated to the remote one.

Furthermore, the local branches are only created by chance, again as a
side-effect of trying to fetch the "special refs".

So, we can't say that we reliably support the use of a branch name.

Update the manual to state that using a branch does not work. Remove
the 'stable' example, as it looked like the name of a stable branch;
instead, replace it with a version string that ressemble a tag.

Fix the layout of the manual by making the version examples an actual
bulleted list.

Note: the above is only entirely true for git. For Mercurial, CVS and
subversion, the status may be mixed, but nonetheless, using branches is
still a bad idea, if at least because it is not reproducible, and
because Buildroot does not even follow the branch. So, we do not
differentiate between the various SCMs, and just flatly state that using
a branch name is not supported.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>