fix the description of the availability threshold

[jingjtang]

closes #807

Prerequisites:

• Unless it is a documentation hotfix it should be merged against the dev branch
• Branch is up-to-date with the branch to be merged with, i.e. dev
• Build is successful
• Code is cleaned up and formatted

Summary

Add more detailed description to the available threshold for Quidel Covidtest signals.

For raw signals, we only report counts larger than or equal to 50.

For smoothed signals,

• when count is smaller than 50 but larger than or equal to 25, we conduct the geographical pooling strategy;
• when count is smaller than 25, we report nothing

[krivard]

👍