Javadoc for Document Context Handling

[peter-lawrey]

✨ What’s new in this PR

A focused documentation pass over Chronicle Wire’s document-context stack plus one tiny API convenience method.

• Brand-new DocumentContext#isOpen() default method
   – a friendly alias that mirrors isNotComplete() and reads more intuitively in client code.

• Thorough, consistent Javadoc across the read/write context family
  (DocumentContext, WriteDocumentContext, ReadDocumentContext,
  Text*DocumentContext, Binary*DocumentContext,
  DocumentContextHolder, WrappedDocumentContext,
  NoDocumentContext, DocumentWritten).

  • first-line “intent sentences” for IDE tooltips
  • clarified life-cycle rules (start/close/reset, rollback, chaining)
  • examples, parameter/return semantics, exception notes
  • internal field explanations and threading notes
  • removal of stale references (delta wire, obsolete flags)

• No behavioural changes – only comments and the small, backwards-compatible addition of isOpen().

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Commit guide

Commit	Message	Highlights
01/10	Create the branch	Adds isOpen() (defaults to isNotComplete()) + stub Javadoc.
02/10	Improve DocumentContext Javadoc (#1021)	Full rewrite; examples & contract clarifications.
03/10	Improve WriteDocumentContext Javadoc (#1022)	Adds start/chain semantics, empty-check docs.
04/10	Clarify ReadDocumentContext behaviour (#1023)	Documents limit/position restore helpers.
05/10	Improve TextReadDocumentContext Javadoc (#1024)	Explains ---/… parsing helpers.
06/10	Improve TextWriteDocumentContext Javadoc (#1025)	Details separator handling, rollback flow.
07/10	Improve BinaryReadDocumentContext Javadoc (#1026)	Modern docs + deprecation note on delta wire.
08/10	Improve BinaryWriteDocumentContext Javadoc (#1027)	Header-writing algorithm spelled out.
09/10	Improve context holder Javadocs (#1028)	Docs for wrapper classes & DocumentWritten.
10/10	Improve Javadoc for context wrappers (#1029)	Final tidy-up (NoDocumentContext, WrappedDocumentContext).