[DOC] update doc threshold image

[Remi-Gau]

Closes #2048

Changes proposed in this pull request:

1. Clarify that voxels with values

• inferior to threshold are set to 0
• supeerior or equal to threshold are kept as is

2. Explain how percentiles are computed.

[github-actions]

👋 @Remi-Gau Thanks for creating a PR!

Until this PR is ready for review, you can include the [WIP] tag in its title, or leave it as a github draft.

Please make sure it is compliant with our contributing guidelines. In particular, be sure it checks the boxes listed below.

• PR has an interpretable title.
• PR links to Github issue with mention "Closes #XXXX"
• Code is PEP8-compliant.
• Changelog or what's new entry in "doc/changes/latest.rst"
• (Bug fixes) There is at least one test that would fail under the original bug conditions.
• (New features) There is at least one unit test per new function / class.
• (New features) The new feature is demoed in at least one relevant example.

We will review it as quick as possible, feel free to ping us with questions if needed.

[codecov]

Codecov Report

Merging #3485 (127efc7) into main (b7e5efd) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #3485   +/-   ##
=======================================
  Coverage   90.97%   90.97%           
=======================================
  Files         133      133           
  Lines       15346    15346           
  Branches     3030     3030           
=======================================
  Hits        13961    13961           
  Misses        818      818           
  Partials      567      567           

Impacted Files	Coverage Δ
nilearn/image/image.py	95.73% <ø> (ø)

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[ymzayek]

LGTM @Remi-Gau! Just minor things. I'd also add a whatsnew entry.

[Remi-Gau]

I'd also add a whatsnew entry.

added something in latest.rst.

[ymzayek]

LGTM otherwise @Remi-Gau

[bthirion]

LGTM, thx.

[ymzayek]

Merging