Extend documentation of FFTESSMethod

[devmotion]

Fixes #13.

[codecov]

Codecov Report

Merging #14 (a7b9998) into main (9f4dcce) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #14   +/-   ##
=======================================
  Coverage   93.72%   93.72%           
=======================================
  Files           8        8           
  Lines         558      558           
=======================================
  Hits          523      523           
  Misses         35       35           

Impacted Files	Coverage Δ
src/ess.jl	94.73% <ø> (ø)

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