[1.6.x] Fixed #21035 -- Changed docs to treat the acronym SQL phonetically.

The documentation and comments now all use 'an' to
refer to the word SQL and not 'a'.

Backport of 4d13cc56de from master

docs/internals/contributing/writing-documentation.txt

@@ -78,6 +78,10 @@ documentation:
 
 * **MySQL**, **PostgreSQL**, **SQLite**
 
+* **SQL** -- when referring to SQL, the expected pronunciation should be
+  "Ess Queue Ell" and not "sequel". Thus in a phrase like "Returns an
+  SQL expression", "SQL" should be preceded by "an" and not "a".
+
 * **Python** -- when referring to the language, capitalize Python.
 
 * **realize**, **customize**, **initialize**, etc. -- use the American