Fix recurring doc test failures with the cat API. (#21561)
Fix recurring doc test failures with the cat API. (#21561)

This failure is due to the fact that we sort on store size, which is cached. So
it might happen that the store size that is taken into account is not the right
one, which makes the indices sorted in the wrong order. This changes the doc
example to sort on the number of docs instead.

Closes #21062

strapdata/elassandra5-rc,strapdata/elassandra5-rc,strapdata/elassandra5-rc,strapdata/elassandra5-rc,strapdata/elassandra5-rc