Move "English" and "Oxford Comma" sections under "Wording" [ci-skip]

The "English" and "Oxford Comma" sections currently have 1 or 2
sentences, which makes it difficult to justify their own top sections.

Moving the "English" section under the "Wording" section makes sense, as
we already have a "Naming" section. We can rename it to "American
English" for easier scanability.
As the "Wording" section also mentions punctuation, we can move the
"Oxford Comma" section there as well.

Author: p8

guides/source/api_documentation_guidelines.md

@@ -98,13 +98,11 @@ used. Instead of:
 * his or hers... use theirs.
 * himself or herself... use themselves.
 
-English
--------
+### American English
 
 Please use American English (*color*, *center*, *modularize*, etc). See [a list of American and British English spelling differences here](https://en.wikipedia.org/wiki/American_and_British_English_spelling_differences).
 
-Oxford Comma
-------------
+### Oxford Comma
 
 Please use the [Oxford comma](https://en.wikipedia.org/wiki/Serial_comma)
 ("red, white, and blue", instead of "red, white and blue").