docs: fix the documentation on the limit ranges for fixed bucket histograms

[hngu]

Fixes #19891.

Description

In this doc (https://druid.apache.org/docs/latest/development/extensions-core/approximate-histograms/#properties) for fixed bucket histograms, it shows that the range [lowerLimit, upperLimit] will be divided into numBuckets intervals of equal size with closed bracket notation. This means inclusivity. However, in the code itself it actually indicates [lowerLimit, upperLimit) which does not include the upperLimit: https://github.com/apache/druid/blob/master/extensions-core/histogram/src/main/java/org/apache/druid/query/aggregation/histogram/FixedBucketsHistogram.java#L389-L396

I am updating the docs to reflect what the code actually does.

Release note

Skipped

This PR has:

• been self-reviewed.
• added documentation for new or modified features or behaviors.
• a release note entry in the PR description.
• added Javadocs for most classes and all non-trivial methods. Linked related entities via Javadoc links.
• added or updated version, license, or notice information in licenses.yaml
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage is met.
• added integration tests.
• been tested in a test Druid cluster.

[FrankChen021]

I have reviewed the code for correctness, edge cases, concurrency, and integration risks; no issues found.

Reviewed 2 of 2 changed files.

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This is an automated review by Codex GPT-5.6-Luna(max)