[DOC] Update FLAMEO documentation and references.

[tsalo]

The old example format was difficult to read.

I chose the references based on the tech report references in the FSL
user guide section describing FLAME
(https://fsl.fmrib.ox.ac.uk/fsl/fslwiki/FEAT/UserGuide#Group_Statistics).

[codecov-io]

Codecov Report

Merging #1961 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1961      +/-   ##
==========================================
+ Coverage   72.52%   72.52%   +<.01%     
==========================================
  Files        1070     1070              
  Lines       54382    54384       +2     
  Branches     7858     7858              
==========================================
+ Hits        39440    39442       +2     
  Misses      13715    13715              
  Partials     1227     1227

Flag	Coverage Δ
#smoketests	72.52% <100%> (ø)	⬆️
#unittests	70.07% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
nipype/interfaces/fsl/model.py	80.61% <100%> (+0.03%)	⬆️

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