Mne-python: [ENH] [Doc] Annotations onset

[annesodub]

Based on the documentation, it is unclear if onsets are expressed in seconds or samples (or anything else).

----------
onset : array of float, shape (n_annotations,)
    Annotation time onsets relative to the ``orig_time``, the starting time
    of annotation acquisition.

[massich]

Indeed, the documentation fails to say that onset should be in seconds.

[makkostya]

Ill do PR.

[massich]

go for it !