RFR: 8214248: (fs) Files:mismatch spec clarifications [v3]

[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.

Brian Burkhalter has updated the pull request incrementally with one additional commit since the last revision:

  8214248: Move mismatch() verbiage to new paragraph

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Changes:
  - all: https://git.openjdk.org/jdk/pull/15350/files
  - new: https://git.openjdk.org/jdk/pull/15350/files/1bd9436a..46384485

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15350&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15350&range=01-02

  Stats: 4 lines in 1 file changed: 2 ins; 0 del; 2 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