[code-reflection] RFR: Terminology updates

[Paul Sandoz]

Tighten terms, esp. around the state transitions of operations and other items that change from unbuilt to built when an associating block transitions from unbuilt (and inaccessible) to built (and accessible).

Add checks when constructing an operation or block reference to ensure only unbuilt values are used, so failing on construction rather than when an operation is appended to a block builder. (Later we can do much better on error messages and even printing out unbuilt models to aid debugging.)

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Commit messages:
 - Updates.
 - Doc updates.

Changes: https://git.openjdk.org/babylon/pull/890/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=890&range=00
  Stats: 362 lines in 11 files changed: 149 ins; 114 del; 99 mod
  Patch: https://git.openjdk.org/babylon/pull/890.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/890/head:pull/890

PR: https://git.openjdk.org/babylon/pull/890