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

[Brian Burkhalter]

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

-------------

Commit messages:
 - 8318677: Add impl note and slightly tweak the test

Changes: https://git.openjdk.org/jdk/pull/16325/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16325&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8318677
  Stats: 5 lines in 2 files changed: 3 ins; 1 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