[MRG] DOC fix missing unit onannotations onset (#4909)
[MRG] DOC fix missing unit onannotations onset (#4909)

* DOC fix missing unit onannotations onset

* improved annotations doc

* fix typo

* fix parameters' order

* fix remove extra space

drammock/mne-python,wmvanvliet/mne-python,cjayb/mne-python,olafhauk/mne-python,pravsripad/mne-python,kingjr/mne-python,adykstra/mne-python,drammock/mne-python,Teekuningas/mne-python,mne-tools/mne-python,larsoner/mne-python,rkmaddox/mne-python,kingjr/mne-python,bloyl/mne-python,mne-tools/mne-python,wmvanvliet/mne-python,teonlamont/mne-python,Eric89GXL/mne-python,larsoner/mne-python,olafhauk/mne-python,mne-tools/mne-python,drammock/mne-python,cjayb/mne-python,bloyl/mne-python,adykstra/mne-python,wmvanvliet/mne-python,teonlamont/mne-python,Teekuningas/mne-python,Teekuningas/mne-python,olafhauk/mne-python,larsoner/mne-python,pravsripad/mne-python,rkmaddox/mne-python,kambysese/mne-python,Eric89GXL/mne-python,kambysese/mne-python,kingjr/mne-python,pravsripad/mne-python