Fix misleading numeric_state trigger documentation

[pnbruckner]

Description:

Existing documentation says numeric_state trigger will fire once when the numeric value of an entity's state "crosses a threshold." However, that's not how the code actually works. It will fire for any state change (from any of the specified entities) if the numeric value of the new state satisfies the above and/or below conditions.

Checklist:

• Branch: Fixes, changes and adjustments should be created against current. New documentation for platforms/components and features should go to next.
• The documentation follow the standards.

[pnbruckner]

I seem to be wrong. I'll close this PR until I'm sure. If I'm right I'll reopen. Otherwise I'll leave it closed.

[pnbruckner]

FWIW, I did look at the code again, and indeed I was wrong. I had originally misunderstood how it worked. I also did some more extensive experimenting to convince myself. But, having said that, there is some behavior that isn't well documented. First, after a restart, if the entity's state satisfies the above and/or below conditions, it doesn't fire. This seems to trip some people up, even though it shouldn't, because with only one state value it can't have "crossed a threshold." But, there is another behavior which is probably unexpected to most. That is, if the first entity state change after restart satisfies the above and/or below conditions, the trigger will fire, even if it doesn't cross a threshold! To be clear, if the state satisfies the conditions after restart, it won't immediately fire, but if it changes to a new state value that also satisfies the conditions, the automation will trigger (again, even though it didn't cross a threshold.) It might be worth describing this behavior. Actually, that behavior should probably get "fixed", but that might break existing automations.