[ticket/11794] Add missing array element commas to docs/coding-guidelines.html

Even though the coding guidelines document prescribes "commas after every
array element", it contains several example code fragments with array elements
not terminated by a comma. This commit fixes that.

PHPBB3-11794
Merge remote-tracking branch 'rechosen/ticket/11794' into develop-olympus

* rechosen/ticket/11794:
  [ticket/11794] Add missing array element commas to docs/coding-guidelines.html