Add doc to prePR command alias

[armanbilge]

Also use session clear instead of reload at the end.

[rossabaker]

I don't know what session clear is, but the lack of doc has bitten me many times over the years.

[armanbilge]

session clear resets any settings modified in the session (in this case, enabling fatal warnings). It's cheaper than reloading the entire build. H/t #210 (comment)