RFR: 8278856: javac documentation does not mention use of Manifest class-path attribute

[Christian Stein]

Please review this improvement to `javac`'s manpage, section **Directory Hierarchies**.

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Commit messages:
 - 8278856: javac documentation does not mention use of Manifest class-path attribute

Changes: https://git.openjdk.org/jdk/pull/22243/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22243&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8278856
  Stats: 6 lines in 1 file changed: 6 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/22243.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22243/head:pull/22243

PR: https://git.openjdk.org/jdk/pull/22243