Explain that reference targets are writable

It might seem like a really obvious point to make but without
emphasizing it, it isn't completely clear. I would like to mention this
in the Branch section as well for how to point a branch at another
commit but I can't see how to smoothly slide it in.
Fix spelling typo
Provide example for Reference.log_append

I would have found this useful when trying to do reflog additions. It
might not be massively complex but examples always help.