Add content doc for split plugins resource file (#3538)

* Add content doc for split plugins resource file

* Fix documentation based on Devin's findings

* Add note why we're not fixing it

core/src/main/resources/jenkins/split-plugins.txt

 # See ClassicPluginStrategy.DETACHED_LIST. As of JENKINS-47634 also used by plugin-compat-tester.
 
+# Columns are: plugin ID, last core release still containing the plugin's functionality, plugin version that's implied.
+
+# Note that all split plugins between and including matrix-auth and jdk-tool incorrectly use the first
+# core release without the plugin's functionality when they should use the immediately prior release.
+# Fixing these retroactively won't help, as the difference only matters to those specific versions.
+
 maven-plugin 1.296 1.296
 subversion 1.310 1.0
 cvs 1.340 0.1