Clarify impact of applying expand to subsequent transformations

Property paths used in a $apply transformation by default may refer to any instances that can be directly or indirectly reached via navigation properties. They are all part of the input set of the transformation.

A preceding expand transformation limits the input set to those navigation paths constructed from the navigation property given as first argument to expand, possibly nested several times, reaching target instances that match the given filter expression.

From the current text this limitation does not become clear.

Furthermore, groupby may follow an expand transformation. The definition of groupby currently refers to an “initial” set, and it should be clarified that this is actually the input set, e.g. the result of the preceding expand.