RFR: 4842457: (bf spec) Clarify meaning of "(optional operation)"

[Brian Burkhalter]

Add some clarifying verbiage about optional operations to the type-specific buffer classes.

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Commit messages:
 - 4842457: (bf spec) Clarify meaning of "(optional operation)"

Changes: https://git.openjdk.org/jdk/pull/13123/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13123&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-4842457
  Stats: 9 lines in 1 file changed: 9 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/13123.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13123/head:pull/13123

PR: https://git.openjdk.org/jdk/pull/13123