annotate `FunctionMixin` methods `_ad_mul` and `_ad_add` (#3841) #5864

Job	Run time
Build Firedrake (real, default)	52m 46s
Build Firedrake (complex, --complex, complex)	46m 31s
Set the Docker release tag	3s
Build Docker containers / docker_env / Build the firedrake-env container	38m 52s
Build Docker containers / docker_complex / Build the firedrake-complex container	22m 57s
Build Docker containers / docker_vanilla / Build the firedrake-vanilla container	21m 56s
Build Docker containers / docker_firedrake / Build the firedrake container	12m 27s
Build Docker containers / docker_docdeps / Build the firedrake-docdeps container	14m 28s
Build Docker containers / docker_jupyter / Build the firedrake-jupyter container	9m 54s
	3h 39m 54s

Provide feedback