RFR: 4842457: (bf spec) Clarify meaning of "(optional operation)" [v3]

[Brian Burkhalter]

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

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

  4842457: Remove vestigial comma; clean up

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Changes:
  - all: https://git.openjdk.org/jdk/pull/13123/files
  - new: https://git.openjdk.org/jdk/pull/13123/files/12e5fe15..93febb09

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13123&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13123&range=01-02

  Stats: 4 lines in 1 file changed: 0 ins; 0 del; 4 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