RFR: 8305762: FileInputStream and FileOutputStream implSpec should be corrected or removed

[Brent Christian]

With the removal of the AltFinalizer mechanism from `FileInputStream` and `FileOutputStream` in [JDK-8192939](https://bugs.openjdk.org/browse/JDK-8192939), this portion of the Implementation Requirement in the class JavaDoc is no longer true:

> If this FileOutputStream has been subclassed and the close() method has been overridden, the close() method will be called when the FileInputStream is unreachable."

The class doc, and the doc for close(), are updated to correctly reflect current behavior, and guidance for subclasses is clarified.

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

Commit messages:
 - remove blank line
 - Clarify that finalization isn't used for cleanup for FIS/FOS, and that Cleaner should be used instead.

Changes: https://git.openjdk.org/jdk/pull/13437/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13437&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8305762
  Stats: 28 lines in 2 files changed: 0 ins; 12 del; 16 mod
  Patch: https://git.openjdk.org/jdk/pull/13437.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13437/head:pull/13437

PR: https://git.openjdk.org/jdk/pull/13437