Clarify Java pageable maxPageSize guidance for TypeSpec samples

[Copilot]

TypeSpec Java guidance did not explain that pageable page sizing belongs on the returned pageable result, causing generated samples/tests to pass maxPageSize directly to client methods and fail compilation.

• Documentation

  • Added explicit guidance in the TypeSpec Java quickstart Samples section.
  • Clarified that Java pageable convenience methods do not accept maxPageSize directly.
  • Directed sample/test authors to use PagedIterable / PagedFlux page iteration APIs instead.

• Correct usage

  client.listItems()
      .iterableByPage(10)
      .forEach(page -> page.getElements().forEach(System.out::println));

  asyncClient.listItems()
      .byPage(10)
      .subscribe(page -> page.getElements().forEach(System.out::println));

• Fixes Use matrix instead of templates for vmImage parallelism #2852

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