Clarify where RequiredProperties have to appear

[ralfhandl]

Capabilities.FilterRestrictionsType/RequiredProperties are defined as

These properties must be specified in the $filter clause

Be more explicit where/how these must be specified.

For example if Foo is required, is this filter expression sufficient?

$filter=Foo eq 'bar' or Baz eq 'qux'

Or do required properties have to appear in every expression of a top-level disjunction?

Imported from ODATA-1611