Improve explicitness of the session history spec

This is mostly an editorial change, which introduces explicit <dfn>s and
links for the various items of a session history entry. However, it also
includes some bugfixes and substantial clarifications. Notable ones:

* Removes the mention of "form data" as being part of a session history
  entry. This was never referenced elsewhere in the spec. It may have
  something to do with form resubmission upon history traversal (which
  seems to be unspecified), or maybe it was subsumed into "persisted
  user state". For now it's best removed.

* Removes a paragraph saying that entries with discarded documents
  should act as if the documents were not discarded. This seems totally
  wrong; re-creation of documents is handled explicitly in the spec.

* Fixes the "URL and history update steps" to not create document-less
  session history entries.

* Symmetrizes and more tightly couples scroll position
  saving/restoration and saving/restoration of other,
  implementation-defined, persisted user state.

* Rewords all the discussion of contiguous, document-sharing session
  history entries for improved clarity.

* Updates all history-related IDL constructs to modern style, including
  adding a "state" value for history.state to return, and creating
  "shared history push/replace state steps" for history.pushState() and
  history.replaceState() to call into.

* Reduces some of the implicit variable-passing, notably for the
  "navigate to a fragment" algorithm.

This helps set the stage for #5767, but does not yet make any of the
changes proposed there.

Author: domenic