HBASE-29989 Update the implementation of splitting in the documentation

[liuxiaocs7]

• see: HBASE-29989

[Copilot]

Pull request overview

Updates the RegionServer splitting documentation to clarify the HDFS directory layout used by newer region split implementations (referencing HBASE-26187), in the context of HBASE-29989.

Changes:

• Adds an informational callout noting that newer split implementations no longer use a parent-region .splits directory.
• Clarifies that the high-level split semantics remain the same, while the HDFS preparation location differs.

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[Copilot]

This note says the numbered split procedure above contains legacy wording about a .splits directory, but the procedure itself still describes creating/using .splits (steps 3/5). To avoid readers following an inaccurate step-by-step flow, consider updating the numbered steps to match the post-HBASE-26187 behavior, or explicitly scoping the numbered procedure to the older implementation and stating which HBase versions/releases the new behavior applies to.

Suggested change

	The numbered split procedure above contains legacy wording that mentions a `.splits` sub-directory under the parent region directory.
	In newer implementations, [HBASE-26187](https://issues.apache.org/jira/browse/HBASE-26187) changed this behavior:
	daughter region directories are created directly under the table directory, rather than under a parent-region `.splits` directory.
	This update does not change the high-level split semantics (references, meta updates, and online transition of daughters), but it changes where temporary/initial daughter files are prepared in HDFS.
	The numbered split procedure above documents the original, pre-HBASE-26187 implementation, in which daughter regions are first created under a `.splits` sub-directory of the parent region directory.
	In newer HBase implementations that include [HBASE-26187](https://issues.apache.org/jira/browse/HBASE-26187), this on-disk layout has changed:
	daughter region directories are created directly under the table directory, rather than under a parent-region `.splits` directory.
	When reading the numbered procedure for such newer versions, interpret any steps that mention creating or using a `.splits` directory as instead preparing and promoting daughter region files directly under the table directory.
	This update does not change the high-level split semantics (references, meta updates, and online transition of daughters); it only affects where temporary/initial daughter files are prepared in HDFS.

[Copilot]

Minor wording/grammar: after the colon, start the next sentence with a capital letter ("Daughter region directories...") and consider avoiding the hyphenated "parent-region" phrasing ("parent region’s .splits directory" reads more naturally).

Suggested change

	daughter region directories are created directly under the table directory, rather than under a parent-region `.splits` directory.
	Daughter region directories are created directly under the table directory, rather than under a parent region's `.splits` directory.

[vaijosh]

LGTM

[PDavid]

Many thanks, looks good to me. 👍

[PDavid]

Looking into the Yetus check it says this:

[INFO] > eslint . && prettier --check "**/*.{md,mdx}"
[INFO] 
[INFO] Checking formatting...
[INFO] [warn] app/pages/_docs/docs/_mdx/(multi-page)/architecture/regionserver.mdx
[INFO] [warn] Code style issues found in the above file. Run Prettier with --write to fix.

Can you please check this?

[liuxiaocs7]

Looking into the Yetus check it says this:

[INFO] > eslint . && prettier --check "**/*.{md,mdx}"
[INFO] 
[INFO] Checking formatting...
[INFO] [warn] app/pages/_docs/docs/_mdx/(multi-page)/architecture/regionserver.mdx
[INFO] [warn] Code style issues found in the above file. Run Prettier with --write to fix.

Can you please check this?

Hi, @PDavid, thanks for pointing out this, I'll try to fix later!