RFR: 8292566: Add reference to the java.nio.file package in java.nio package documentation

[Brian Burkhalter]

Add a phrase about `java.nio.file` to the sentence in the `java.nio` package documentation describing its subpackages.

-------------

Commit messages:
 - 8292566: Add reference to the java.nio.file package in java.nio package documentation

Changes: https://git.openjdk.org/jdk/pull/9910/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9910&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8292566
  Stats: 4 lines in 1 file changed: 1 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/9910.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9910/head:pull/9910

PR: https://git.openjdk.org/jdk/pull/9910