Consolidate from source upgrade notes

doc/update/upgrading_from_source.md

@@ -405,6 +405,13 @@ If all items are green, then congratulations, the upgrade is complete!
 
 ## Version specific upgrading instructions
 
+Upgrading versions might need some manual intervention. For more information,
+[check the version you are upgrading to](index.md#version-specific-upgrading-instructions)
+for additional steps required for all GitLab installations, and for
+steps that apply to installations from source.
+
+<!--  move all notes to main upgrade page -->
+
 This section contains upgrading instructions for specific versions. When
 present, first follow the upgrading guidelines for all versions. If the version
 you are upgrading to is not listed here, then no additional steps are required.