RFR: 8306431: File.listRoots method description should be re-examined [v6]

[Nagata-Haruhito]

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

Nagata-Haruhito has updated the pull request incrementally with three additional commits since the last revision:

 - Merge branch 'File_javadoc' of https://github.com/Nagata-Haruhito/jdk into File_javadoc
 - Drop exist to available
 - Merge branch 'master' into File_javadoc

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Changes:
  - all: https://git.openjdk.org/jdk/pull/13526/files
  - new: https://git.openjdk.org/jdk/pull/13526/files/787f71a1..7ba2460a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13526&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13526&range=04-05

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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