RFR: 8306431: The documentation of File.listRoots should be modifid after JDK-8208077

[Nagata-Haruhito]

I fixed File.listRoots description.
* remove "the insertion or ejection of removable media"
* change "available" to "existing"

Please review this change.

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Commit messages:
 - 8306431: The documentation of File.listRoots should be modifid after JDK-8208077

Changes: https://git.openjdk.org/jdk/pull/13526/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13526&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8306431
  Stats: 5 lines in 1 file changed: 0 ins; 1 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/13526.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13526/head:pull/13526

PR: https://git.openjdk.org/jdk/pull/13526