Revert "Adding a bit more documentation to mw.loader.using"

This isn't relevant or specific to mw.loader.using. All of ResourceLoader
revolves around this idea. If two separate modules have a common
dependency, that's not loaded twice either. Whether they were passed to
loader.using(), loader.load(), or as dependencies, it doesn't matter.

The callback executes when the requires modules are ready. The
documentation never said it would load them on demand, so this bit of
supposed clarification only leads functions as noise in my opinion.

Perhaps we need to clarify elsewhere in the documentation for mw.loader
as a whole this concept, but not in this method in this way.

This reverts commit 657fab8d8388a475d850d84d8244da5e0ccfa8e7.

Change-Id: I30b3d6769f72aaab47c0b987bbb58898612206bd
Merge "Revert "Adding a bit more documentation to mw.loader.using""