docs($interval): remind the developer to destroy their intervals

It is essential that users of `$interval` destroy the interval when they are finished.
Otherwise you can get memory leaks.
Often `$intervals` are used in directives or controllers and developers don't think
about what happens when the component is destroyed.
If a directive/controller scope is destroyed, then the $interval should be destroyed as well.
This could cause some issues with developers who assume that the interval will be cleared
for them when the scope is destroyed.

Closes #5377

I believe that the library could/should handle this as well, but thats another issue.