Improve javadoc in 26 files.

Addresses:
BUKKIT-1643, BUKKIT-1868, BUKKIT-1846, BUKKIT-2632, BUKKIT-3196,
BUKKIT-3187, BUKKIT-3198, BUKKIT-3200, BUKKIT-3201 and BUKKIT-3417.
Don't try listing something that may not exist. Fixes BUKKIT-3471

The player name may not be provided, in which case the command will
fail hard.
Only use TravelAgent if supplied; Addresses BUKKIT-3466

If there is no TravelAgent assigned, it can not be used.
Merge remote-tracking branch 'upstream/master'