[jdk17] RFR: 8266631: StandardJavaFileManager: getJavaFileObjects() impl violates the spec

[Jan Lahoda]

[a copy of https://github.com/openjdk/jdk/pull/4360 for openjdk/jdk17]

This improves javadoc/specification of several StandardJavaFileManager methods, as requested in these bugs:
https://bugs.openjdk.java.net/browse/JDK-8266631
https://bugs.openjdk.java.net/browse/JDK-8266596
https://bugs.openjdk.java.net/browse/JDK-8266591
https://bugs.openjdk.java.net/browse/JDK-8266590

The CSR is here:
https://bugs.openjdk.java.net/browse/JDK-8268260

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

Commit messages:
 - Improving javadoc as suggested on the review.
 - 8266631: StandardJavaFileManager: getJavaFileObjects() impl violates the spec

Changes: https://git.openjdk.java.net/jdk17/pull/56/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk17&pr=56&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266631
  Stats: 21 lines in 1 file changed: 7 ins; 0 del; 14 mod
  Patch: https://git.openjdk.java.net/jdk17/pull/56.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk17 pull/56/head:pull/56

PR: https://git.openjdk.java.net/jdk17/pull/56