RFR: 8364213: (bf) Improve java/nio/Buffer/GetChars.java test comments

[Brett Okken]

Address code review comments which came in post integration/merge of https://github.com/openjdk/jdk/pull/26282.

Update/clarify test summary tag
Document the values generated for arguments
Move the endian-ness qualifier in the test descriptor string to not be first
Change test method names to more closely match arg names of method under test

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Commit messages:
 - 8364213: (bf) Improve java/nio/Buffer/GetChars.java test comment

Changes: https://git.openjdk.org/jdk/pull/26512/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26512&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8364213
  Stats: 29 lines in 1 file changed: 12 ins; 0 del; 17 mod
  Patch: https://git.openjdk.org/jdk/pull/26512.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26512/head:pull/26512

PR: https://git.openjdk.org/jdk/pull/26512