[doc] fix sphinx warnings

12 jobs for ffilelist in 11 minutes and 21 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Build
passed #35762
conda-linux
build_linux_27

00:00:50

passed #35764
conda-linux
build_linux_34

00:00:53

passed #35766
conda-linux
build_linux_35

00:00:48

passed #35767
conda-macosx
build_macosx_27

00:01:21

passed #35765
conda-macosx
build_macosx_34

00:01:25

passed #35763
conda-macosx
build_macosx_35

00:01:18

 
  Test
passed #35772
conda-linux
test_linux_27

00:00:40

96.0%
passed #35770
conda-linux
test_linux_34

00:05:15

96.0%
passed #35768
conda-linux
test_linux_35

00:01:06

96.0%
passed #35769
conda-macosx
test_macosx_27

00:02:46

96.0%
passed #35771
conda-macosx
test_macosx_34

00:04:05

96.0%
passed #35773
conda-macosx
test_macosx_35

00:04:45

96.0%