This makes it possible to fully "override" some base `Targets` filter
with another (e.g. user-supplied) filter. Without some way to obtain the
default, only explicit targets can be overridden (via `IntoIter` and
friends).

Ideally the method would be named `default`, corresponding to
`with_default`, however this conflicts with `Default::default` and so
would be a breaking change (and harm ergonomics). `default_level` seemed
a better name than `get_default`, since "getters" of this style are
generally considered unidiomatic[citation needed].

Co-authored-by: Eliza Weisman <eliza@buoyant.io>

The new documentation includes more complete examples showing defaults
set by `with_default` or parsed, as well as examples of no defaults and
a specifically set `LevelFilter::OFF` default. You may also notice that
2nd person language has been removed.