Code-quality review: documentation-only improvements

[linkdata]

Documentation-only changes from the core-package quality review. Stacked on #80
(based on the code-quality-review branch); merge after #80. No behavior change —
comments only; build, vet, staticcheck, golangci-lint and the full -race /
-tags debug -race suites are unaffected.

• Element queue helpers (SetAttr/SetInner/...): replace the misleading
  "call only during JawsRender/JawsUpdate" line with the accurate deferred-delivery
  contract and point to Request.Dirty for the event-driven path; expand the
  Element.queue chokepoint doc.
• contracts.go: fix the dangling [Element.Context]/[Element.Session]
  doc-links to [Request.Context]/[Request.Session]; give Container.JawsContains
  a leading synopsis sentence; note the deleted-Element no-op on Renderer.JawsRender
  and Updater.JawsUpdate.
• eventhandler.go: document ErrEventHandlerPanic's Unwrap semantics and that
  the recorded Type is the UI object type (may differ from the handler that panicked).
• request.go: split Request.Get/Set synopses + add [Session.Get]/[Session.Set]
  links; document that AlertError also logs; note that Request methods are not
  nil-safe (unlike Session); record the recycle-window reasoning on appendDirtyTags
  and the retained-back-pointer/frozen invariant in clearLocked.
• broadcast.go: document distributeDirt's deliberate pending-buffer and
  recycle-window behavior.
• jaws.go / session.go: note the CSPRNG panic condition on NewRequest/NewSession.
• errvalueunchanged.go: name the JawsSet/JawsSetPath convention and why the
  sentinel lives in this package.
• element.go: link the public [CallEventHandlers] entry from the appendHandlers note.