Recommend running firedrake-clean after updating (#4404)

Author: JHopeCollins

docs/source/install.rst

@@ -221,6 +221,12 @@ to set the right environment variables and then run::
 
      $ pip install --upgrade firedrake
 
+Previously generated code may not be compatible with a newer
+Firedrake installation, and may crash with cryptic messages.
+We recommend removing any cached code after updating by running::
+
+     $ firedrake-clean
+
 Updating PETSc
 ~~~~~~~~~~~~~~