clarify the relationship of the various path expression types

[ralfhandl]

The section 14.4.1 Path Expressions lists various different path exptessions

The sections 14.4.1.4 to 6 mention the relationship between the built-in types Edm.PropertyPath, Edm.AnyPropertyPath, or Edm.ModelElementPath but never makes it explicit what the relationship is.
Under ModelElementPath is states
"specify the built-in types Edm.ModelElementPath"
Under Navigation Property Path it states
"specify the built-in types Edm.NavigationPropertyPath, Edm.AnyPropertyPath, or Edm.ModelElementPath.

Which seems to indicate the ModelElementPath is more specific.
Can this be explained in a more comprehensive way ?

Proposal

#228

Imported from ODATA-1496