Updates to docs for branch command

Made a few readability, elaboration, typo and grammar fixes to the
branch command documentation

Sorry if this looks like a wall of red. Most f the changes are pretty small: moving a word here or there, fixing a typo or some grammar.

In a couple places I tried to elaborate a bit, for example on why you can't init the trunk after having cloned just a branch. I also added a hint about how you might go about figuring out the `--tfs-parent-branch` in TFS2008. 

Let me know if I should split this into smaller commits or if there's a single change you want to revert. If this is agreeable, I'll take a pass through some of the other docs later while I'm looking for some places to jump in on actual code :)