vimrc: add comment to my "delete buffer & go to previous tab" map

Spell out what the mapping is doing.
vimrc: add <leader>tl to go to last active tab

Makes it easier to switch between two tabs when there are a bunch open.
Instead of having to memorize their tab numbers, just use this command.
Taken from
http://stackoverflow.com/questions/2119754/switch-to-last-active-tab-in-vim