Update documentation to describe exposed filters with depth

I spent hours last night (stupidly) trying to figure out how to make SHS give me an exposed filter that would allow me to select a parent term in a hierarchy and show all the children. The documentation only mentions adding a new filter using the term reference field. I noticed in #1901924: Support for taxonomy filter has taxonomy terms (with depth) (and before that in #1903380: Views Exposed - Possibility to add -any- option instead of -none-.) that SHS adds an option for "Content: Has taxonomy terms (with depth; Simple hierarchical select)"

The documentation should probably reflect this. It's a great feature!