mock: document public API in collector module

There has been interest around publishing `tracing-mock` to crates.io
for some time. In order to make this possible, documentation and some
code clean up is needed.

This change adds documentation to the collector module itself and to all the
public APIs in the module. This includes doctests on all the methods
that serve as examples.

Additionally the implementation for the `Expect` struct has been moved
into the module with the definition, this was missed in #2369.

Author: hds