Improve documentation for probes

[aamster]

There is documentation lacking for much of the probes data. The following are the fields which need more documentation

Channel

• valid_data
• local_index
• probe_vertical_position
• probe_horizontal_position
• manual_structure_acronym
• manual_structure_id
• anterior_posterior_ccf_coordinate
• dorsal_ventral_ccf_coordinate
• left_right_ccf_coordinate
• impedance
• filtering

Unit

• peak_channel_id
• local_index
• cluster_id
• quality
• firing_rate
• isi_violations
• presence_ratio
• amplitude_cutoff
• mean_waveforms
• spike_amplitudes
• spike_times
• isolation_distance
• l_ratio
• d_prime
• nn_hit_rate
• nn_miss_rate
• max_drift
• cumulative_drift
• silhouette_score
• waveform_duration
• waveform_halfwidth
• PT_ratio
• repolarization_slope
• recovery_slope
• amplitude
• spread
• velocity_above
• velocity_below
• snr