Clarify covidcast documentation about diff-based vs confirmation-based issues

[krivard]

From Logan in: cmu-delphi/covidcast-indicators#1362

I soft-expected an issue-query to return only data with changes to entries (other than to issue itself and lag), for two reasons:

• ?covidcast::covidcast_signal describes issue-queries as "[f]etch[ing] only data that was published or updated ("issued") on these [issues]", which might leave this impression.
• storage efficiency-wise, this would make sense; query efficiency-wise, I'm not sure but I'd guess that it'd probably help, especially if there is an index over source,signal,geo_value,time_value,issue.

We should be clear in the documentation that the covidcast data history occasionally includes records that confirm (match) a previous value, rather than being restricted to only additions and updates.

If we feel the need to be expansive, here is the full explanation:

Confirmation-type issues are expected under certain circumstances:

1. Certain sources always use confirmation-type issues (fb-survey, doctor-visits, hospital-admissions)
2. All issues for all sources prior to 2020-07-16 were confirmation-type, since that's how we generated the initial set of versions when versioning was launched. Indicators were switched to diff-based issues one at a time afterward (with poor bookkeeping as to the timing of each switch)
3. With the exception of the bigint patches, all data patches applied prior to November 2021 were confirmation-type, since setting up the machinery for a diff-type patch is fairly complicated and we only recently established software to assist with the process