RFR: 8318677: (ch) Add implNote about minBufferCap to main variant of Channels.newWriter [v4]

[Brian Burkhalter]

> Add an `implNote` about the `minBufferCap` parameter of the three-arg `Channels.newWriter` and slightly tweak the test.

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

  8318677: Improve the clarity of the spec of minBufferCap in newWriter

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Changes:
  - all: https://git.openjdk.org/jdk/pull/16325/files
  - new: https://git.openjdk.org/jdk/pull/16325/files/60ba5c0f..a48c2ece

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16325&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16325&range=02-03

  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/16325.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16325/head:pull/16325

PR: https://git.openjdk.org/jdk/pull/16325