mediawiki.mixins.less: Correctly embed images in .background-image-svg

Using the /* @embed */ annotation twice for the same property
(background-image) confuses the LESS compiler, resulting in one of the
images not being embedded.

Let's use the embeddable() and embed() MediaWiki LESS functions,
like in the .background-image mixin.

And let's extend and clean up the documentation while we're here.

Bug: 61941
Change-Id: I39ea2004b9db34d096eed08b3ffdfcaf8d5fcd7d