Avoid that one filetype overrides the other's swaplist.

With only one global g:swap_lists, setting the swaplist for one filetype overrides the previous filetype's list. This breaks swapping with multiple filetypes in parallel (e.g. a PHP and CSS file); the last opened filetype "wins".

Instead, a buffer-local variable should be used. Each buffer has its own swaplist (according to its filetype / manual :SwapList) definition then.
Minor: Remove duplicate variable definition and unused var.
Also make g:swap_xml_matchit buffer-local.

As this configuration is filetype-specific, too.
Update documentation with b:swap_lists.

Also, I've removed the duplicate multi-word swap list comment from the plugin, as this is already explained at :help swapit-multiword. Probably, a lot more unnecessary stuff should be cut from the plugin's comment header.