RFR: JDK-8276772: Refine javax.lang.model docs [v2]

[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.

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 four additional commits since the last revision:

 - Update history of preview feature.
 - Merge branch 'master' into JDK-8276772
 - Merge branch 'master' into JDK-8276772
 - JDK-8276772: Refine javax.lang.model docs

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6286/files
  - new: https://git.openjdk.java.net/jdk/pull/6286/files/d455dce0..57b3982c

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6286&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6286&range=00-01

  Stats: 586 lines in 23 files changed: 490 ins; 23 del; 73 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