Cache now returns null when using get(cacheID) with invalid ID (#441)

* Cache now returns `null` when using get(cacheID) where the ID is invalid.

* Documented `get(id)`

* Adjusted version in conf.py to 8.0.0

docs/docs_cache.rst

@@ -25,6 +25,10 @@ get(key, value)
 
 Returns a contained object where ``object[key] == value``. Also works if value is a regex or a function. Returns the first object found that matches the criteria.
 
+get(value)
+
+Returns a contained object where ``object["id"] == value``. Shorthand for ``get("id", value)``. Returns ``null`` if ID is not found.
+
 getAll(key, value)
 ~~~~~~~~~~~~~~~~~~