Undocumented monstergroups.json things

[Hirmuolio]

Is your feature request related to a problem? Please describe.
There are multiple undocumented features in the monstergroups.json

https://github.com/CleverRaven/Cataclysm-DDA/blob/master/doc/JSON_INFO.md#monster-groups

Describe the solution you'd like
The following should be documented. I have heard of some of them before and took a quick look at the code. Just need confirming and clarifying some details and I'll add these to the doc.

• "is_animal": Different spawn densities for wild animals (?)
• "override": (default false) When set to true will override already existing group of same name.
• "cost_multiplier": Mislabeled as multiplier in the doc?
• "starts": This entry becomes active at this time (time unit?)
• "ends": This entry becomes inactive at this time (time unit?)
• "replace_monster_group": (bool) Evolve this monster group
• "new_monster_group_id": Evolve this group into this group
• "replacement_time": Evolve this monster group at (time unit?)
• "is_safe": ?
• "freq_total": Change from x/1000 to x/freq_total probablitiy
• "auto_total": Set chance to 1/total of frequencies. So always spawns a monster from the list and never the default monster.

Describe alternatives you've considered

Additional context

[stale]

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