Added scriptcommands addmonsterdrop and delmonsterdrop (Missing documentation)

[Edit by Haru: This commit has been amended in order for it to work
properly in Hercules, and to correct some flaws of the original version,
according to what I said in my review of issue #191. Credits to the
original author have been moved to an "Original idea by" comment. Commit
authorship is preserved. Documentation will be added in a follow-up
commit]

Signed-off-by: Haru <haru@dotalux.com>
Added documentation for addmonsterdrop/delmonsterdrop

Follow-up to 7ddcff9ae2c45659605c649a0af8bb25539ecade
Resolves #191

Signed-off-by: Haru <haru@dotalux.com>