[Doc]: Add more info about how temporal averaging is performed and how weights are generated

[tomvothecoder]

Describe your documentation update

Related to #594

Solutions for the time being would be to update documentation to note that:

- Weights are determined from the difference in the bounds
- Data is grouped into the labelled time point for averaging operations
- If one time point spans across the time intervals that you are averaging into, then weights are not properly assigned
- get_time_bounds generally assumes data with frequencies of annual, monthly, daily, or sub-daily (I thought of this while writing this issue)
- Perhaps other disclaimers I haven't thought of