RFR: JDK-8281169: Expand discussion of elements and types [v2]

[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 with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains five additional commits since the last revision:

 - Iteration of spec update.
 - Merge branch 'master' into JDK-8281169
 - Checkpoint intermediate state.
 - Merge branch 'master' into JDK-8281169
 - JDK-8281169: Expand discussion of elements and types

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/15097/files
  - new: https://git.openjdk.org/jdk/pull/15097/files/9a06ca1a..350a5d1f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15097&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15097&range=00-01

  Stats: 18487 lines in 793 files changed: 8431 ins; 4965 del; 5091 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