Describe the issue linked to the documentation

This is the current documentation for parameter include_boundaries in the function check_scalar:

"left": only min_val is included in the valid interval;

"right": only max_val is included in the valid interval;

"both": min_val and max_val are included in the valid interval;

"neither": neither min_val nor max_val are included in the valid interval.

Suggest a potential alternative/fix

Add the following interval notation in the documentation for the function check_scalar:

• "left" [ , )
• "right" ( , ]
• "both" [ , ]
• "neither" ( , )

Note for Beginners

Here are notes on how to get started:

1. Read the documentation: https://scikit-learn.org/stable/developers/contributing.html
2. Read notes on Git: https://www.dataschool.io/how-to-contribute-on-github/
3. Watch videos: https://www.youtube.com/playlist?list=PLBKcU7Ik-ir-b1fwjNabO3b8ebs9ez5ga
4. Go through the code base, explore it and find the file(s) where the edits need to be made.
5. Read Stack Overflow's "How do I ask a good question?"
6. Be inclusive in your communications: inclusive language
   • Do not use salutations such as "Hello Sir", or "Hey guys" (Why: these terms exclude women and non-binary people.)
   • Do not use salutations such as "Hello Sir/Madam" (Why: these terms assume gender is binary, and they exclude non-binary people.)

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