Add a warning for Priority in availability rules & add explanation of Availability order in docs

[Valentina R]

Too many people use the priority numbers and get very weird behaviors and get lost and do not understand how the system works.

It would be great to add a sentence to the hover with something like this: Unless you have a very complicated system, do not use the Priority but use the order of rules to set the priority'.

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It would also be very good to add this explanation (or similar) here:

The rules go like this (assuming you are not changing the Priority number and leaving it at default. Default is 10 for all rules created in WP and 5 for events created via gCal) for a specific product:

1. Global rules (if any) are applied.
2. Product rules (if any) are applied on top of that.
3. Staff rules (if any) are applied on top of that.
4. Global gCal rules (if any) are applied on top of that.
5. Staff gCal rules (if any) are applied on top of that.

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[Željan Topić]

Added inside documentation on the website