RFR: 8307409: Refactor usage examples to use @snippet in the java.nio packages [v7]

[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: Make indentation consistent; convert straggling HTML entities

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Changes:
  - all: https://git.openjdk.org/jdk/pull/13870/files
  - new: https://git.openjdk.org/jdk/pull/13870/files/a531b06b..1bb11251

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13870&range=06
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13870&range=05-06

  Stats: 53 lines in 11 files changed: 0 ins; 0 del; 53 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