CHNG documentation should indicate sample size is not provided

[capnrefsmmat]

The CHNG documentation (and documentation for any other source with suppressed sample size) should document that the sample_size column is left deliberately blank, and give whatever reason we can share for that.

[krivard]

@LeonLu2 these are all covidcast signals, so you know where to find the documentation sources for them already.

Match disclaimer and reason with doctor visits: "Standard errors and sample sizes are not available for this data source."

• Doctor Visits
• CHNG
• Hospital Admissions
• Google Search Trends

Disclaimer and reason: "Standard errors and sample sizes are not applicable to these metrics."

• Data Strategy and Execution Workgroup (DSEW)
• JHU
• USAFacts
• Department of Health and Human Services (HHS)
• NCHS Mortality

[krivard]

Fixed in #961