TRUE UNIQUE flag for overmap specials

[vorpal-void]

Is your feature request related to a problem? Please describe.

It is somewhat a problem I've run into while playing Cata++ mod, more on that here: https://github.com/Noctifer-de-Mortem/nocts_cata_mod/issues/258 Basically, after getting answer from mod author and reading through OVERMAP.md I've found out that game seems to be lacking the option of placing really true unique overmap specials. Either that, or their placement is not too well documented, more on that in additional context.

Describe the solution you'd like

A separate flag like [TRUE UNIQUE] that will allow specific overmap special to be placed only once per world generation.

Describe alternatives you've considered

Perhaps, some more options to control what's happening on mapgen, but that's unrelated to current suggestion.

Additional context

Reading through overmap.cpp I've found out that in the void overmap::place_specials( overmap_special_batch &enabled_specials ) there is some [ENDGAME] flag that seems to be used for "id": "Central Lab" I've not been into yet, and that flag is not documented. I'm not very good with the code, but from the comments I've got an impression that this flag does exactly what I'm suggesting:

Line 4330: // Add instances_placed from truly mandatory batch to regular batch, to avoid double placement Line 4331: // Endgame specials were placed above, don't let UNIQUE overwrite that

If that so, docs could use an update on that.