[STY] Minor doc formatting

[Remi-Gau]

• Closes have to use noqa to pacify flake8 when blacken docs reformats long lines  #4016

Changes proposed in this pull request:

• unindent some code blocks
• remove extra backticks in class attributes doc

Started to solve #4016 and improve visual rendering, also ended up homogenzing Attributes in classes doc string to match numpy styles

[github-actions]

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[Remi-Gau]

Before:

After:

[codecov]

Codecov Report

Merging #4072 (e50a05b) into main (9fcc19d) will decrease coverage by 0.10%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #4072      +/-   ##
==========================================
- Coverage   91.60%   91.50%   -0.10%     
==========================================
  Files         143      143              
  Lines       16072    16072              
  Branches     3339     3339              
==========================================
- Hits        14722    14707      -15     
- Misses        804      820      +16     
+ Partials      546      545       -1     

Flag	Coverage Δ
macos-latest_3.10	91.49% <ø> (ø)
macos-latest_3.11	91.49% <ø> (ø)
macos-latest_3.12	91.49% <ø> (ø)
macos-latest_3.8	91.45% <ø> (ø)
macos-latest_3.9	91.46% <ø> (ø)
ubuntu-latest_3.10	?
ubuntu-latest_3.11	?
ubuntu-latest_3.12	91.49% <ø> (ø)
ubuntu-latest_3.8	91.45% <ø> (ø)
ubuntu-latest_3.9	91.46% <ø> (ø)
windows-latest_3.10	91.43% <ø> (ø)
windows-latest_3.11	91.43% <ø> (ø)
windows-latest_3.12	91.43% <ø> (ø)
windows-latest_3.8	91.39% <ø> (ø)
windows-latest_3.9	91.40% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
nilearn/_utils/docs.py	91.86% <ø> (ø)
nilearn/connectome/connectivity_matrices.py	94.50% <ø> (ø)
nilearn/connectome/group_sparse_cov.py	81.58% <ø> (ø)
nilearn/decoding/decoder.py	96.92% <ø> (ø)
nilearn/decoding/space_net.py	87.32% <ø> (ø)
nilearn/decomposition/_base.py	94.87% <ø> (ø)
nilearn/decomposition/_multi_pca.py	100.00% <ø> (ø)
nilearn/decomposition/canica.py	91.66% <ø> (ø)
nilearn/decomposition/dict_learning.py	89.06% <ø> (ø)
nilearn/plotting/displays/_projectors.py	98.24% <ø> (ø)
... and 4 more

... and 6 files with indirect coverage changes

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[bthirion]

LGTM so far.