Super minor, but it was bugging me.

There was a missing closing paren in the docs.

git_fetch_options was missing from the API docs because it lacked a
documentation comment above the struct declaration.
I used the git_checkout_options docstring as a template.

Also fixes a typo in git_remote_prune_refs (remote, not reamote).

Fallback describes the mechanism, while unspecified explains what the
user is thinking.

A remote's URLs are now modified according to the url.*.insteadOf
and url.*.pushInsteadOf configurations. This allows a user to
replace URL prefixes by setting the corresponding keys. E.g.
"url.foo.insteadOf = bar" would replace the prefix "bar" with the
new prefix "foo".