Document need to opt-in to tags/modifiers for `antlers: true` fields

[duncanmcclean]

statamic/cms#14092

[efc]

Please be clear in the documentation about what the default allowed tags and modifiers are and whether adding tags or modifiers overwrites these defaults or adds to the defaults.

[edalzell]

Can this please be prioritized? I just spent over an hour trying to figure out what the correct config is to bring our sites back up.