[DOC] Improve docstrings of maskers

[ymzayek]

Addresses part of #3384 .

Changes proposed in this pull request:

• Add docs about attributes to multi subject maskers
• Make docstrings consistent across maskers
• Use standard docs where possible

[github-actions]

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[codecov]

Codecov Report

Merging #3414 (29dfe52) into main (cdebb69) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##             main    #3414    +/-   ##
========================================
  Coverage   91.57%   91.57%            
========================================
  Files         139      139            
  Lines       16575    16577     +2     
  Branches     3061     3233   +172     
========================================
+ Hits        15178    15180     +2     
  Misses       1394     1394            
  Partials        3        3            

Impacted Files	Coverage Δ
nilearn/maskers/multi_nifti_labels_masker.py	100.00% <ø> (ø)
nilearn/maskers/multi_nifti_maps_masker.py	100.00% <ø> (ø)
nilearn/maskers/multi_nifti_masker.py	97.72% <ø> (ø)
nilearn/maskers/nifti_labels_masker.py	93.71% <ø> (ø)
nilearn/maskers/nifti_maps_masker.py	97.68% <ø> (ø)
nilearn/maskers/nifti_masker.py	91.83% <ø> (+0.05%)	⬆️
nilearn/maskers/nifti_spheres_masker.py	96.57% <ø> (ø)
nilearn/_utils/docs.py	93.65% <100.00%> (+0.05%)	⬆️

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[bthirion]

LGTM, thx !

[NicolasGensollen]

LGTM, thanks @ymzayek !