diff --git a/clang/docs/ClangPlugins.rst b/clang/docs/ClangPlugins.rst
--- a/clang/docs/ClangPlugins.rst
+++ b/clang/docs/ClangPlugins.rst
@@ -200,3 +200,14 @@
   PluginASTAction::ActionType getActionType() override {
     return AddAfterMainAction;
   }
+
+Interaction with ``-clear-ast-before-backend``
+----------------------------------------------
+
+If the main AST action is codegen, having any plugins that run after the
+codegen action automatically turns off ``-clear-ast-before-backend``.
+``-clear-ast-before-backend`` reduces peak memory by clearing the Clang AST after
+generating IR and before running IR optimizations. Use
+``CmdlineBeforeMainAction`` or ``AddBeforeMainAction`` as ``getActionType`` to
+run plugins before the codegen action and still have access to peak memory
+savings from ``-clear-ast-before-backend``.