RFR: 8308016: Use snippets in java.io package

[Brian Burkhalter]

Replace `<pre>{@code ...}</pre>` patterns and the like with `{@snippet lang=java : ...>`.

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Commit messages:
 - 8308016: Use snippets in java.io package

Changes: https://git.openjdk.org/jdk/pull/13957/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13957&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8308016
  Stats: 178 lines in 21 files changed: 27 ins; 7 del; 144 mod
  Patch: https://git.openjdk.org/jdk/pull/13957.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13957/head:pull/13957

PR: https://git.openjdk.org/jdk/pull/13957