user-manual: clarify difference between tag and branch

Explain the difference (well, one of the differences) between a tag
and a branch.

Signed-off-by: "J. Bruce Fields" <bfields@citi.umich.edu>

Documentation/user-manual.txt

@@ -304,6 +304,9 @@ v2.6.13
 ...
 ------------------------------------------------
 
+Tags are expected to always point at the same version of a project,
+while branches are expected to advance as development progresses.
+
 Create a new branch pointing to one of these versions and check it
 out using gitlink:git-checkout[1]: