[Feature] Add documentation on differences among aggregations across different grains

Copied from fivetran/dbt_ad_reporting #92.

Is there an existing feature request for this?

[X] I have searched the existing issues

Describe the Feature

Some customers have brought up concerns on why sums are different across different grains, for example why spend is not the same summed up across the ad level versus the campaign level.

Example from Joe regarding a recent customer inquiry:

"The reason for this discrepancy is due to not all ads being served at the ad level. There are some ads that are only served at the ad group/campaign/etc. levels. This means that you can have ads sent at the campaign level and that counts towards your total spend. However, since the ad was never sent at an individual ad level, it will not be included.
This was a large reason for us breaking the ad reporting package into separate hierarchical end models. Because we found when we only used the ad level, we were missing data as some ads were not served at an ad level, but were served at a campaign level."

We should add more context like above to our ad packages READMEs as a proactive measure.

Describe alternatives you've considered

No response

Are you interested in contributing this feature?

[X] Yes.
[ ] Yes, but I will need assistance and will schedule time during your office hours for guidance.
[ ] No.

Anything else?