Copy remaining style guidance to authoring guide

These two items of style were the only bits present in the style guide
and missing from the authoring guide.  Let's copy them over, since
they are true of the Reference, and this will let us delete the style
guide entirely.

Author: traviscross

docs/authoring.md

@@ -126,6 +126,8 @@ All this does is apply a CSS class to the blockquote. You should define the colo
 
 Idioms and styling to avoid:
 
+* Use American English spelling.
+* Use Oxford commas.
 * Avoid slashes for alternatives ("program/binary"); use conjunctions or rewrite it ("program or binary").
 * Avoid qualifying something as "in Rust"; the entire reference is about Rust.