[doc] fix sphinx warnings

12 jobs for doc in 7 minutes and 46 seconds (queued for 10 seconds)
Status Job ID Name Coverage
  Build
passed #35521
conda-linux
build_linux_27

00:02:40

passed #35523
conda-linux
build_linux_34

00:02:34

passed #35525
conda-linux
build_linux_35

00:02:32

passed #35526
conda-macosx
build_macosx_27

00:03:52

passed #35524
conda-macosx
build_macosx_34

00:03:50

passed #35522
conda-macosx
build_macosx_35

00:03:55

 
  Test
passed #35531
conda-linux
test_linux_27

00:00:33

94.0%
passed #35529
conda-linux
test_linux_34

00:00:35

94.0%
passed #35527
conda-linux
test_linux_35

00:00:34

94.0%
passed #35528
conda-macosx
test_macosx_27

00:00:56

94.0%
passed #35530
conda-macosx
test_macosx_34

00:00:57

94.0%
passed #35532
conda-macosx
test_macosx_35

00:00:56

94.0%