RFR: 8278923: Document Klass::is_loader_alive

[Coleen Phillimore]

This trivial change just adds a comment to Klass::is_loader_alive.

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Commit messages:
 - change a word
 - 8278923: Document Klass::is_loader_alive

Changes: https://git.openjdk.org/jdk/pull/9400/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9400&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8278923
  Stats: 5 lines in 1 file changed: 4 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/9400.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9400/head:pull/9400

PR: https://git.openjdk.org/jdk/pull/9400