Docs: [no-unnecessary-condition] `checkTypePredicates` section: mention assumptions of valid type predicates

[kirkwaiblinger]

Before You File a Documentation Request Please Confirm You Have Done The Following...

• I have looked for existing open or closed documentation requests that match my proposal.
• I have read the FAQ and my problem is not listed.

Suggested Changes

The checkTypePredicates feature relies on the assumption that the type predicate is "if and only if". While this is a requirement of a well-formed type predicate in TS, that may not be common knowledge for TS users. Also, this may actually not be the case for the asserts variants... See https://discord.com/channels/1026804805894672454/1088474511759917106/1506050671399538919. But we should probably wait for some more input from the TS side before addressing that in the docs.

Affected URL(s)

https://typescript-eslint.io/rules/no-unnecessary-condition/#checktypepredicates

Additional Info

No response