[Doc] Bitmap explanation is incorrect

[believerHan]

Why I'm doing:

What I'm doing:

Fixes #issue

What type of PR is this:

• BugFix
• Feature
• Enhancement
• Refactor
• UT
• Doc
• Tool

Does this PR entail a change in behavior?

• [] Yes, this PR will result in a change in behavior.
• No, this PR will not result in a change in behavior.

If yes, please specify the type of change:

• Interface/UI changes: syntax, type conversion, expression evaluation, display information
• Parameter changes: default values, similar parameters but with different default values
• Policy changes: use new policy to replace old one, functionality automatically enabled
• Feature removed
• Miscellaneous: upgrade & downgrade compatibility, etc.

Checklist:

• I have added test cases for my bug fix or my new feature
• This pr needs user documentation (for new or modified features or behaviors)
  • I have added documentation for my new feature or new function
• This is a backport pr

Bugfix cherry-pick branch check:

• I have checked the version labels which the pr will be auto-backported to the target branch
  • [] 3.2
  • 3.1
  • 3.0
  • 2.5

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[imay]

Hi, the document is correct.
We want users to create a bitmap index for high cardinality columns.
Suppose creating a bitmap index for a sex column that just has male/female, half of the records will be read. In that case, the query performance can not be accelerated by the index.

[believerHan]

I looked at the 2.5 version documentation and Wikipedia, and it says bitmap is suitable for low cardinality. The example on your official website is also a low-base example.