More rigorous definition of $<request identifier> references

Sections 11.7.4 and 11.7.6 of OData-Protocol introduce "references" of the form $<request identifier> but do not rigorously explain how these are to be evaluated.

These references should be defined in analogy with the $root URL segment (in section 11.7.4) or $this references (in section 11.7.6). They should also appear in the ABNF.