RFR: 7057369: (fs spec) FileStore getUsableSpace and getUnallocatedSpace could be clearer

[Brian Burkhalter]

Attempt to clarify when the values returned for a `FileStore`s usable and unallocated space are initialized and likely to be valid.

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Commit messages:
 - 7057369: (fs spec) FileStore getUsableSpace and getUnallocatedSpace could be clearer

Changes: https://git.openjdk.org/jdk/pull/17271/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17271&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-7057369
  Stats: 11 lines in 1 file changed: 1 ins; 0 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/17271.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17271/head:pull/17271

PR: https://git.openjdk.org/jdk/pull/17271