Improve documentation of Collectible add/remove callbacks

Repentogon adds, as far as I can tell, 3 new callbacks for when a collectible gets added or removed: MC_PRE_ADD_COLLECTIBLE, MC_POST_ADD_COLLECTIBLE and MC_POST_TRIGGER_COLLECTIBLE_REMOVED.

The documentation feels a bit lacking, though; in general it would be helpful to know when these get called, especially in cases like when rerolling with a D4 or after the usage of 'M? If so, it could allow dynamic addition/removal of callbacks that depend on a specific item being equipped, not just an active that gets used.

In addition, MC_PRE_ADD_COLLECTIBLE speaks of "accept[ing] a table of parameters" while referring to an allowed return value. The table also lists table or CollectibleType, while the text also mentions a boolean return value being possible. Would be nice to fix the inconsistencies there.

Start with the documentation entries for MC_PRE_ADD_COLLECTIBLE, MC_POST_ADD_COLLECTIBLE, and MC_POST_TRIGGER_COLLECTIBLE_REMOVED, then verify their invocation timing for rerolls and after M?. Document when each callback runs and correct the MC_PRE_ADD_COLLECTIBLE parameter and return-value descriptions. Done means the callback timing and accepted table, CollectibleType, and boolean forms are consistent.