docs: show phase signature for builders (#44067)

lib/spack/docs/build_systems/autotoolspackage.rst

@@ -147,6 +147,15 @@ example, the ``bash`` shell is used to run the ``autogen.sh`` script.
    def autoreconf(self, spec, prefix):
        which("bash")("autogen.sh")
 
+If the ``package.py`` has build instructions in a separate
+:ref:`builder class <multiple_build_systems>`, the signature for a phase changes slightly:
+
+.. code-block:: python
+
+   class AutotoolsBuilder(AutotoolsBuilder):
+      def autoreconf(self, pkg, spec, prefix):
+         which("bash")("autogen.sh")
+
 """""""""""""""""""""""""""""""""""""""
 patching configure or Makefile.in files
 """""""""""""""""""""""""""""""""""""""