Document the idle transaction and waitForChildren behaviour for automatic UI transactions

[marandaneto]

Since more SDKs want to implement this, it'd make sense to be documented.
Flutter PR getsentry/sentry-dart#643
It needs to be generic enough since it also depends on the framework.

[marandaneto]

cc @denrase

[AbhiPrasad]

getsentry/sentry-java#1523 (comment) is a short description of how it is implemented for browser javascript.

[AbhiPrasad]

We are experimenting with idle transaction behaviour in JS, see getsentry/sentry-javascript#4531 for more details.

[philipphofmann]

Fixed with #766. Duplicate of #647.