RFR: 8318572: GenShen: Fix error in log message for ingored old-has-grown triggers

[Kelvin Nilsen]

Fix messages in log and user guidance.

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

Commit messages:
 - Change log_info to log_debug for ignored old-has-grown triggers
 - Fix log message and user guidance

Changes: https://git.openjdk.org/shenandoah/pull/345/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=345&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8318572
  Stats: 11 lines in 2 files changed: 1 ins; 1 del; 9 mod
  Patch: https://git.openjdk.org/shenandoah/pull/345.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/345/head:pull/345

PR: https://git.openjdk.org/shenandoah/pull/345