[DOC-14585][AI] Correct the Disk i/o optimized thread count

[TimLFletcher]

Fixes DOC-14585.

@daniel Owen reported this, so he is the right person to confirm the number.

What changed

modules/manage/pages/manage-settings/general-settings.adoc:224, in Data Settings.

-Couchbase Server sets the number of threads equal to the number of CPU cores on the node.
+Couchbase Server sets the number of threads to twice the number of CPU cores on the node.

Only occurrence of the claim on this branch.

Open questions for the reviewer

1. Confirm the multiplier is 2x. The ticket states it plainly and comes from KV engineering, but nothing in this repository states a multiplier either way, so there is no second source.
2. Does the same correction apply to release/7.6? The ticket cites the 7.6 page, and 7.6 carries the same claim at line 181 ("The number of threads allocated is equal to the number of CPU cores for the node"). It is a different Server generation, so the allocation may genuinely have differed. If it is also 2x there, that branch needs the same change.
3. release/8.5 has the identical sentence to this branch and needs the same change. Happy to raise that PR on confirmation, or now if you would rather not wait.

Affected branches

Branch	Line	Wording	This PR
release/8.0	224	"sets the number of threads equal to the number of CPU cores on the node"	yes
release/8.5	same	identical	no, see question 3
release/7.6	181	"The number of threads allocated is equal to the number of CPU cores for the node"	no, see question 2

Verification

PASS[docs-server @ DOC-14585 vs baseline release/8.0]: no new build diagnostics.

The build confirms the page still renders. It cannot confirm the number, which is question 1.

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