RFR: 8307409: Refactor usage examples to use @snippet in the java.nio packages

[Brian Burkhalter]

Convert the `<pre>{@code ...}</pre>` and some other patterns to `{@snippet ..}`.

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Commit messages:
 - 8307409: Refactor usage examples to use @snippet in the java.nio packages

Changes: https://git.openjdk.org/jdk/pull/13870/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13870&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8307409
  Stats: 88 lines in 6 files changed: 0 ins; 0 del; 88 mod
  Patch: https://git.openjdk.org/jdk/pull/13870.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13870/head:pull/13870

PR: https://git.openjdk.org/jdk/pull/13870