Attempt to clarify environment marker evaluation

Preparation for the release of packaging 25.1 revealed multiple
deficiencies in the specification of environment marker evaluation.
Review of the proposed amendments to resolve those deficiencies
highlighted multiple other problems, including some dating from the
original PEP 508 specification:

* other pages still referencing PEP 508 instead of the living spec
* direct reference to PEP 685 instead of the core metadata spec
* the "extra" special case not being properly defined
* lacking guidance to tool developers regarding what should be
  considered errors to disallow entirely vs issues to work around

Inspired by the initial PR at #1971