RFR: 8327046: (fs) Files.walk should be clear that depth-first traversal is pre-order

[Pavel Rappo]

Please review this clarification to the specification of the two overloads of `java.nio.file.Files.walk`. No tests are added, as I believe the existing `StreamTest.testWalk*` methods already test the order sufficiently. I also took this opportunity to fix a few minor typos.

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.org/jdk/pull/18063/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18063&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8327046
  Stats: 17 lines in 2 files changed: 2 ins; 0 del; 15 mod
  Patch: https://git.openjdk.org/jdk/pull/18063.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18063/head:pull/18063

PR: https://git.openjdk.org/jdk/pull/18063