RFR: 7057369: (fs spec) FileStore getUsableSpace and getUnallocatedSpace could be clearer [v2]

[Brian Burkhalter]

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

Brian Burkhalter has updated the pull request incrementally with one additional commit since the last revision:

  Address reviewer comments

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Changes:
  - all: https://git.openjdk.org/jdk/pull/17271/files
  - new: https://git.openjdk.org/jdk/pull/17271/files/7d10b049..0df648db

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17271&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17271&range=00-01

  Stats: 8 lines in 1 file changed: 0 ins; 1 del; 7 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