Document percieved memory leak

QJSEngine doesn't call collectGarbage upon deletion, because this
operation can be expensive and in most cases the JS engine is deleted
during application shutdown, when the memory can be deallocated cheaper.

This can lead to an apparent memory leak if multiple engines are created
and destroyed in the same process. Documenting how to avoid this.

Change-Id: Ie02cb8bedc8821e0bb04dfa52a5a2d65b4b085a3
Reviewed-by: Christopher Adams <chris.adams@jollamobile.com>