Clean up docs around $wgUseSharedUploads

Clean up docs around $wgUseSharedUploads

* Remove references to these old configuration variables
  from documentation. Rather than wgLocalFileRepo referring to
  these for more information, move the docs there, and point
  there from the old variables.

* Move all their declarations in DefaultSettings.php together,
  and be in the same order as used in Setup.php.

* Add missing @var type and @since version docs.

Bug: T25122
Change-Id: I4eb9490ed4e978b2c3954de0593a9d0d3c5fe73c