RFR: 8361972: Clarify the condition of System.console() about standard input/output

[Naoto Sato]

This accompanies the fix for [JDK-8361613](https://bugs.openjdk.org/browse/JDK-8361613), which restricts `System.console()` to return a `Console` instance only when both standard input and output are connected to a terminal. The change here is solely a specification clarification and tightening of the `java.io.Console` javadoc to reflect this behavior. We are separating the spec clarification because the fix itself may be backported to prior LTS releases without requiring a Maintenance Review process.

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

Commit messages:
 - initial commit

Changes: https://git.openjdk.org/jdk/pull/26299/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26299&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8361972
  Stats: 3 lines in 1 file changed: 1 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/26299.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26299/head:pull/26299

PR: https://git.openjdk.org/jdk/pull/26299