RFR: 8336462: ConcurrentSkipListSet Javadoc incorrectly warns about size method complexity

[Viktor Klang]

Removes some of the old wording around the algorithmic complexity of ConcurrentSkipListSet::size() while still retaining the warning around the accuracy of the returned result.

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Commit messages:
 - Improving the JavaDoc of ConcurrentSkipListSet::size() to reflect current reality

Changes: https://git.openjdk.org/jdk/pull/20388/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20388&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8336462
  Stats: 8 lines in 1 file changed: 0 ins; 5 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/20388.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20388/head:pull/20388

PR: https://git.openjdk.org/jdk/pull/20388