Missing some documentation on auto_indent

[mwchase]

The Settings Reference documentation on auto_indent talks about what happens when the user presses "Enter", but auto_indent has some other behavior: when the user presses "Tab" on an empty line, the default keybinding replaces it with the "Reindent" command.

I'm mostly just entering this issue because I want the information to be online somewhere. (I don't know why it does this.)