[doc] fix sphinx warnings

12 jobs for minimal_load in 3 minutes and 14 seconds (queued for 4 seconds)
Status Name Job ID Coverage
  Build
passed build_linux_27 #35320
conda-linux

00:01:19

passed build_linux_34 #35322
conda-linux

00:01:17

passed build_linux_35 #35324
conda-linux

00:01:16

passed build_macosx_27 #35325
conda-macosx

00:01:59

passed build_macosx_34 #35323
conda-macosx

00:01:54

passed build_macosx_35 #35321
conda-macosx

00:01:55

 
  Test
passed test_linux_27 #35330
conda-linux

00:00:48

90.0%
passed test_linux_34 #35328
conda-linux

00:00:50

90.0%
passed test_linux_35 #35326
conda-linux

00:00:48

90.0%
passed test_macosx_27 #35327
conda-macosx

00:01:10

90.0%
passed test_macosx_34 #35329
conda-macosx

00:01:04

90.0%
passed test_macosx_35 #35331
conda-macosx

00:01:01

90.0%