RFR: JDK-8281169: Expand discussion of elements and types [v4]

[Joe Darcy]

> The goal is this change is to convey to the reader an accurate "vibe" for the dichotomy between elements and type in the javax.lang.model API rather than a primer on category theory (projection-embedding pairs, etc.).

Joe Darcy has updated the pull request incrementally with one additional commit since the last revision:

  Implement review feedback.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/15097/files
  - new: https://git.openjdk.org/jdk/pull/15097/files/fe197d6c..1fd0e2da

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15097&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15097&range=02-03

  Stats: 5 lines in 3 files changed: 0 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/15097.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15097/head:pull/15097

PR: https://git.openjdk.org/jdk/pull/15097