RFR: 8310682: No package-info (and @since) for package jdk.nio.mapmode

[Brian Burkhalter]

Add `package-info.java` for package `jdk.nio.mapmode`.

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Commit messages:
 - 8310682: No package-info (and @since) for package jdk.nio.mapmode

Changes: https://git.openjdk.org/jdk/pull/14628/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14628&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310682
  Stats: 33 lines in 1 file changed: 33 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/14628.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14628/head:pull/14628

PR: https://git.openjdk.org/jdk/pull/14628