References passed to the callback function of `git_reference_foreach`
are expected to be owned by the callback. As such, they are never being
freed by `git_reference_foreach`, but will have to be freed by the
caller. This small detail is never mentioned in the function's
documentation, though, making it easy to get wrong. Document this to
make it discoverable.

The signature for the reflog is not something which changes
dynamically. Almost all uses will be NULL, since we want for the
repository's default identity to be used, making it noise.

In order to allow for changing the identity, we instead provide
git_repository_set_ident() and git_repository_ident() which allow a user
to override the choice of signature.

This makes them show up in the reference, even if the text itself isn't
the most descriptive.

These have been found with

    grep -Przon '\n\ntypedef struct.*?\{' -- include
    grep -Przon '\n\ntypedef enum.*?\{' -- include

If it's not documented, it doesn't show up in the docs (and we really
should document, anyway).