RFR: 4774868: (fc spec) Unclear spec for FileChannel.force

[Brian Burkhalter]

Add verbiage to `FileChannel.force()` indicating that if the FileChannel is derived from a FileOutputStream or a RandomAccessFile using `getChannel()`, then any modifications made to the channel's file by methods invoked on the parent object will also be forced.

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Commit messages:
 - 4774868: (fc spec) Unclear spec for FileChannel.force

Changes: https://git.openjdk.java.net/jdk/pull/7207/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7207&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-4774868
  Stats: 5 lines in 1 file changed: 3 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7207.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7207/head:pull/7207

PR: https://git.openjdk.java.net/jdk/pull/7207