Update include documentation (mainly amxmodx.inc)

This series of commits updates some includes (mainly amxmodx.inc) to have proper comments. amxmodx.inc has received a complete overhaul while ham_const.inc, amxconst.inc, hlsdk_const.inc, message_const.inc and cellarray.inc only needed some touch-ups to make the new docgen happy and be a little more consistent. cellarray.inc also contains the updated ArrayCreate documentation referenced in PR #107.

If I continue my quest I will do so on a different branch, so this can be merged as soon as we have ironed out all mistakes and errors.

Sorry for the largely meaningless commit messages, but I didn't feel like creating single commits for well over 200 functions. :cat: