Improve function docs for Repository::tag_foreach

[cyqsimon]

Previously the documentation did not make it clear the effect of the return value of cb. This PR clarifies that, alongside a minor rewording of existing docs.

Ref:

https://libgit2.org/docs/reference/main/tag/git_tag_foreach_cb.html

git2-rs/src/tagforeach.rs

Lines 18 to 43 in 1d9f4d4

	/// c callback forwarding to rust callback inside `TagForeachData`
	/// see original: <https://libgit2.org/libgit2/#HEAD/group/callback/git_tag_foreach_cb>
	pub(crate) extern "C" fn tag_foreach_cb(
	name: *const c_char,
	oid: *mut git_oid,
	payload: *mut c_void,
	) -> c_int {
	panic::wrap(|| unsafe {
	let id: Oid = Binding::from_raw(oid as *const _);
	let name = CStr::from_ptr(name);
	let name = name.to_bytes();
	let payload = &mut *(payload as *mut TagForeachData<'_>);
	let cb = &mut payload.cb;
	let res = cb(id, name);
	if res {
	0
	} else {
	-1
	}
	})
	.unwrap_or(-1)
	}

[ehuss]

Thanks!

I notice that it is a little unfortunate that this doesn't do any error handling. Though I suppose an error would probably only happen with a corrupt repo or other catastrophic situation. A callback that panics will also silently drop the panic and just stop iteration.