RFR: 8307409: Refactor usage examples to use @snippet in the java.nio packages [v6]

[Brian Burkhalter]

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

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

  8307409: Update after scanning java.nio source tree

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Changes:
  - all: https://git.openjdk.org/jdk/pull/13870/files
  - new: https://git.openjdk.org/jdk/pull/13870/files/03e2ed3d..a531b06b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13870&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13870&range=04-05

  Stats: 215 lines in 28 files changed: 26 ins; 0 del; 189 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