docs: Further update to multi-swap README.md to clarify the use of multi-swap and OOB swaps

[scrhartley]

Description

Added an more explicit alternative to using multi-swap using hx-select-oob, so it's easier for non-experts to see the similarities between the two. Users often struggle with hx-swap-oob since they don't realise it's for the response html, and hx-select-oob is the more equivalent attribute as implied by the pre-existing issue.

Additionally, removed a clarification only useful in relation to the htmx v1 extension, whose documentation exists in the htmx repo, not here.

Htmx version: 2.x.x
Used extension(s) version(s): 2.x.x

Corresponding issue: #124, follow up to PR #148

Testing

n/a

Checklist

• I have read the contribution guidelines
• I ran the test suite locally (npm run test) and verified that it succeeded

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