Fix the documentation for git_cred_acquire_cb

[rcjsuen]

If you look at the documentation for git_cred_acquire_cb, you will see that the format is completely off with everything at the bottom with its return value and parameters undocumented. Compare this with git_diff_progress_cb where its parameters and return value is correctly documented because of its proper use of tags in the header file.

The documentation for git_cred_acquire_cb should be using standard @param and @return tags so that the documentation will be generated correctly.

[ethomson]

Thanks!