Add a small note on acronyms not to expand

Author: sirosen

documentation/style-guide.rst

@@ -71,6 +71,7 @@ such as "for example" or "that is."
 In general, the first time an acronym is used on a page, spell it out.
 Prefer to write out the full term and follow it with the acronym in parentheses.
 For example, write "Basic Multilingual Plane (BMP)".
+Commonly understood acronyms, such as "HTML" and "UTF-8", should not be expanded.
 
 
 Charged terminology to avoid