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.

(cherry picked from commit 92db96cb09a3191add7464220777ca9cfcf97e27)
Edit description of Chao estimates in the diversity vignette

(cherry picked from commit d689ed87db134f5da95c24485a2f54e4847b7703)
Formatting of diversity vignette

(cherry picked from commit d24db8b233328d961ea77c65e46f9531b5ee418e)
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 The used form is unpublished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.

(cherry picked from commit b7f92c6fb41288c3e441e14acc044bb0ce748e26)
Document the exact Chao bia-corrected variance estimator

(cherry picked from commit d8253316fc983d4d0ba5a2910a0079b8cffd8852)
Use only the new "exact" variance estimator for Chao bias-reduced

(cherry picked from commit 9e49c56035bd0ac91b4bd23f2900d57ba4361ff9)
Edit diversity-vegan vignette

Be explicit about extended species richness only concerning
the number of unseen species and not the total number of species.

(cherry picked from commit b06ec8328d3083804d2a84ad4b74b67cab41d428)
Edit diversity-vegan

(cherry picked from commit 79eb46ece1cac5081f50d4bd84b0c4e8899b2b1c)
tweaks

(cherry picked from commit 685e5795e02deb75c78803f2f1c2e82ef3c49663)