Improve documentation

In libgit2 nomenclature, when we need to verb a direct object, we name
a function `git_directobject_verb`.  Thus, if we need to init an options
structure named `git_foo_options`, then the name of the function that
does that should be `git_foo_options_init`.

The previous names of `git_foo_init_options` is close - it _sounds_ as
if it's initializing the options of a `foo`, but in fact
`git_foo_options` is its own noun that should be respected.

Deprecate the old names; they'll now call directly to the new ones.

The git_describe_options in git_describe_commit and
git_describe_workdir and the git_describe_format_options in
git_describe_format are optional and can be NULL. State this in the
documentation to make people's lives easier when calling these
functions.

Signed-off-by: Remy Suen <remy.suen@gmail.com>

optios -> options