Update clone function docs and params

[AleKoure]

Small PR to update documentation of the clone() function.

1. Clarified that the url parameter also accepts a local repository. While this isn't explicitly documented in libgit2, it might be a good point to clarify in wrappers like git2r.
2. The @params url and local_path are set by default equal to NULL but they are mandatory.

[stewid]

Thanks for the contribution.