kernel-log-check: mention sof_local_extra_kernel_ignores[] at the top

`sof_local_extra_kernel_ignores[] was added by commit 83f5e41
but it is not useful if it is unknown. Mention it at the top.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>

Author: marc-hb

tools/sof-kernel-log-check.sh

@@ -10,6 +10,15 @@
 # Ignoring errors is very dangerous for reasons detailed below so please
 # read this and think twice before making changes in this file.
 
+# You don't need to change this file for errors specific to unusual
+# configurations or unusual devices. Instead, define a
+# sof_local_extra_kernel_ignores=( ... ) array in either
+# /etc/sof/local_config.bash or ${SCRIPT_HOME}/case-lib/local_config.bash
+# This is also a convenient way to stage and test future `ignore_str` changes
+# without git acrobatics.
+#
+# Detailed explanation  in commits 83f5e4190e9f21bf and f94cdc772758692
+
 # Error types
 # -----------
 #