Fix some comments in options.rpy, and a typo in the docs.

These commits fix two typos in options.rpy (persisten -> persistent, themes.roundrect -> theme.roundrect) as well as remove the no longer true information about theme.roundrect being the only theme currently supported, as Ren'Py now comes with a huge variety of themes. They also fix a scary typo in the docs, naming the android.keystore file android.keyring in one place, which isn't the correct name, and a not-so-scary one with a missing K.