RFR: JDK-8281169: Expand discussion of elements and types

[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.).

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Commit messages:
 - JDK-8281169: Expand discussion of elements and types

Changes: https://git.openjdk.org/jdk/pull/15097/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15097&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8281169
  Stats: 102 lines in 1 file changed: 101 ins; 0 del; 1 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