-Doc: Improve Doxygen markup with a few links to a constant and functions.


git-svn-id: http://svn.openttd.org/trunk@26684 6aa0318a-3be1-0310-93fa-89fd2396df07
-Fix: Tighten parameter bound checks on GSCargoMonitor functions, and return -1 on out-of-bound parameters.


git-svn-id: http://svn.openttd.org/trunk@26685 6aa0318a-3be1-0310-93fa-89fd2396df07