Show short tool descriptions

[Emerson Murphy-Hill]

I'm not sure if we implemented this, but I think we said we wanted short tool descriptions. For example, for ISNA, we can just use the description from office support:

Returns the logical value TRUE if value refers to the #N/A (value not available) error value; otherwise, it returns FALSE. https://support.office.com/en-in/article/ISNA-function-a6e8078e-3cf7-4f08-a041-1461e2e55205

I think we said this would appear below the function application name but above the screencasts.

[Aniket Lawande]

Implemented this. Will hand in a version to test.