user-manual: clarify difference between tag and branch
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" <51738506c1b2ccb0761f23bdc612c93babf738ea@citi.umich.edu>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git