RFR: 8329644: Discuss expected visitor evolution patterns in javax.lang.model.util

[Joe Darcy]

Provide more concrete discussion of how the preview visitors are expected to evolve for different categories of language changes.

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Commit messages:
 - Fix whitespace.
 - JDK-8329644: Discuss expected visitor evolution patterns in javax.lang.model.util

Changes: https://git.openjdk.org/jdk/pull/18804/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18804&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8329644
  Stats: 259 lines in 1 file changed: 259 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/18804.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18804/head:pull/18804

PR: https://git.openjdk.org/jdk/pull/18804