Search pagination: approximateResultCount is certainly wrong

Created by: slimsag

After https://github.com/sourcegraph/sourcegraph/pull/4796 is merged, approximateResultCount in paginated requests will only reflect the paginated request itself -- not taking into account prior repos in prior paginated requests with the cursor. Either the documentation needs to reflect this, or it needs to take into account prior requests. This probably doesn't make sense to handle until there is a UI in place to see if this field is required/useful in practice.