RFR: JDK-8276772: Refine javax.lang.model docs

[Joe Darcy]

Pass over javax.lang.model docs for some minor cleanups and improvements.

Before pushing, I'll do any needed copyright updates and reflow paragraphs.

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Commit messages:
 - JDK-8276772: Refine javax.lang.model docs

Changes: https://git.openjdk.java.net/jdk/pull/6286/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6286&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276772
  Stats: 18 lines in 10 files changed: 12 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6286.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6286/head:pull/6286

PR: https://git.openjdk.java.net/jdk/pull/6286