tough: clarify targets vs all_targets functions

[webern]

Repository::targets was created before we implemented delegated targets, then we added Repository::all_targets.

On pub fn targets in tough/src/lib.rs; it says Returns the list of targets present in the repository. which doesn't clarify about delegated targets, and 4 methods down is all_targets, which includes delegated, and says return a vec of all targets including all target files delegated by targets.

Maybe those two should be next to each other, with docstrings that make the difference clearer.