RFR: 8295919: java.security.MessageDigest.isEqual does not adhere to @implNote [v2]

[Kevin Driver]

> Fix JDK-8295919 by updating the javadoc to explain that a null or zero-length `digestb` will also result in a short-circuit response

Kevin Driver has updated the pull request incrementally with one additional commit since the last revision:

  rephrased per code review

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/15933/files
  - new: https://git.openjdk.org/jdk/pull/15933/files/0ee35d9d..17ed6c71

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15933&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15933&range=00-01

  Stats: 5 lines in 1 file changed: 0 ins; 1 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/15933.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15933/head:pull/15933

PR: https://git.openjdk.org/jdk/pull/15933