[MRG] update user guide, docstrings and comments from undersampling methods

[solegalli]

What does this implement/fix? Explain your changes.

Updates docstrings, comments and user guide from undersampling methods, except those from ENN, RENN, AllKNN and NCL which are done / mentioned in a separate PR.

Any other comments?

[solegalli]

#850 contains updates to ENN, RENN and AllKNN

[solegalli]

#764 a parameter needs changing, so skipping update of docstrings ofr NCL here

[solegalli]

still need to update NearMiss

[solegalli]

@glemaitre @chkoar ready for review

[codecov]

Codecov Report

Merging #853 (d52ec25) into master (edf6eae) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #853   +/-   ##
=======================================
  Coverage   98.69%   98.69%           
=======================================
  Files          93       93           
  Lines        6063     6063           
  Branches      508      508           
=======================================
  Hits         5984     5984           
  Misses         78       78           
  Partials        1        1           

Impacted Files	Coverage Δ
...rototype_selection/_condensed_nearest_neighbour.py	100.00% <ø> (ø)
...n/under_sampling/_prototype_selection/_nearmiss.py	98.63% <ø> (ø)
...pling/_prototype_selection/_one_sided_selection.py	100.00% <ø> (ø)
...nder_sampling/_prototype_selection/_tomek_links.py	100.00% <ø> (ø)

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[solegalli]

@glemaitre would you like me to reopen and update this PR?

[glemaitre]

Yep I could review it. As mentioned in #850, could you split in several PRs to have small changes.