Clarify Units.spike_times and Units.obs_intervals documentation

[h-mayorquin]

Summary of changes

The spike_times and obs_intervals doc strings in the Units type now specify that times are relative to the session reference time (i.e. session_start_time or, if defined, timestamps_reference_time). The spike_times doc states that values should be stored in ascending order, and obs_intervals adds that intervals should be non-overlapping. These are documentation-only changes that codify existing assumptions without breaking any files.

Checklist

For all schema changes:

• Add release notes for the PR to docs/format/source/format_release_notes.rst.
• Have you included the relevant issue number using "Fix #XXX" notation where XXX is the issue number? By including "Fix #XXX" you allow GitHub to close issue #XXX when the PR is merged.
• Make sure that hdmf-common-schema points to the latest release and not the latest commit on the main branch.