RFR: 8214248: (fs) Files:mismatch spec clarifications

[Brian Burkhalter]

Clarify that `Files::mismatch` does not access the file system if both paths locate the same file. Add package documentation about file operations on directories being implementation specific.

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Commit messages:
 - 8214248: (fs) Files:mismatch spec clarifications

Changes: https://git.openjdk.org/jdk/pull/15350/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15350&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8214248
  Stats: 11 lines in 2 files changed: 8 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/15350.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15350/head:pull/15350

PR: https://git.openjdk.org/jdk/pull/15350