Add documentation for partial position search

[mattbruv]

Describe the feature

To me, this was one of the more interesting features that compelled me to download and try this app. There was no documentation I could find which explains how you it works or how to use it. I figured it out eventually and in reality it is buried under four different tabs (Analysis -> Database -> Local -> Options -> Partial). For a while I thought the program just didn't have it.

It also appears that however the partial position search works, it's incapable of searching for partial positions in the lichess databases, so it would be nice to note its limitations as well.