dev-manual: Updated branch descriptions for better manual maintenance

dev-manual: Updated branch descriptions for better manual maintenance

The "Repositories, Tags, and Branches" section had some wording
that described how branches are maintained in the poky repo that
made it hard to maintain in the doc from release to release.  I had
a list of past releases by name and as such would have to update
it every release to make it reflect the last set.  Well... this
is not the best way to do this and proved out as it slipped through
the cracks for the YP 1.8 release.  Consequently, I recast the text
so it is "release proof".

(From yocto-docs rev: b6e27d06df58a43bb767e9616ccf3c0711e83cf6)

Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>