manual: cvs: document that a branch or a date can be used

Also instead of using the generic word "timestamp" use the word "tag".

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

docs/manual/adding-packages-generic.txt

@@ -280,7 +280,9 @@ information is (assuming the package name is +libfoo+) :
      Only anonymous pserver mode is supported.
      +LIBFOO_SITE+ 'must' contain the source URL as well as the remote
      repository directory. The module is the package name.
-     +LIBFOO_VERSION+ is 'mandatory' and 'must' be a timestamp.
+     +LIBFOO_VERSION+ is 'mandatory' and 'must' be a tag, a branch, or
+     a date (e.g. "2014-10-20", "2014-10-20 13:45", "2014-10-20
+     13:45+01" see "man cvs" for further details).
   ** +git+ for retrieving source code from a Git repository. Used by
      default when +LIBFOO_SITE+ begins with +git://+. The downloaded
      source code is cached as with the +svn+