RFR: JDK-8294546: document where javac differs when invoked via launcher and ToolProvider

[Jonathan Gibbons]

Please review a doc-only change to specify the differences between using different ways to invoke javac.

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Commit messages:
 - JDK-8294546: document where javac differs when invoked via launcher and ToolProvider

Changes: https://git.openjdk.org/jdk/pull/10566/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10566&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294546
  Stats: 75 lines in 1 file changed: 75 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/10566.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10566/head:pull/10566

PR: https://git.openjdk.org/jdk/pull/10566