Use simpler expression for the variance of Chao extrapolation

The new expression uses lower power or one multiplication less than
the old one. The old one directly lifted from the literature, and
it is mathematically equal to the new one.
Edit description of Chao estimates in the diversity vignette
Formatting of diversity vignette
More exact derivation of approximate variance of bias-reduced Chao

The common texts (and EstimateS web page) use approximate estimator
for bias-corrected Chao estimate. This uses derivation for the basic
estimator but replaces terms with those of bias corrected form
(a1^2 -> a1*(a1-1), 2*a2 -> 2*(a2+1)). The used form is unpublished
as such, but it follows exactly the derivation as given in Chiu et al.,
web appendix. The numerical differences between forms are tiny: in
BCI the maximum difference is ca. 0.02%, and in the 'mite' data up to
12% -- it seems that difference vanishes as the sample size (no. of
individuals) increases.
Document the exact Chao bia-corrected variance estimator
Use only the new "exact" variance estimator for Chao bias-reduced
Edit diversity-vegan vignette

Be explicit about extended species richness only concerning
the number of unseen species and not the total number of species.
Edit diversity-vegan
tweaks
Merge branch 'chao-edit'