git branch documentation improvements

Hello all,

Today I read the git branch documentation and noticed a few things, mostly 
style and consistency related, that could easily be improved, so I did just 
that.
Please consider merging the attached patch if it looks good, or tell me what 
you don't like about it.
The most significant change is renaming <start-point> (or is it 
<startpoint>...) to <branch-head> because even I as a relative beginner know 
that a branch is defined by its (movable) head, and <start-point> *does* 
actually specify the new branch head if I'm not mistaken.

Cheers,
Andreas Hartmetz