doc: Document WITH_EXTERNAL_LIBMULTIPROCESS build option better #32271

1. 
   ryanofsky commented at 8:35 pm on April 14, 2025: contributor

   This includes a cmake documentation change suggested #31741 (review) and another change to mention the option in markdown documentation
2. doc: Document WITH_EXTERNAL_LIBMULTIPROCESS build option better

   This includes a cmake documentation change suggested
   https://github.com/bitcoin/bitcoin/pull/31741#discussion_r2039716492
   and another change to mention the option in markdown documentation
   
   Co-authored-by: Cory Fields <cory-nospam-@coryfields.com>

   9ccee9cd02
3. 
   DrahtBot commented at 8:35 pm on April 14, 2025: contributor

   The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

   Code Coverage & Benchmarks

   For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/32271.

   Reviews

   See the guideline for information on the review process.

   Type	Reviewers
   ACK	hebasto, TheCharlatan

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   Conflicts

   Reviewers, this pull request conflicts with the following ones:

   • #31802 (Add bitcoin-{node,gui} to release binaries for IPC by Sjors)

   If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

4. DrahtBot added the label Docs on Apr 14, 2025
5. hebasto approved
6. 
   hebasto commented at 8:43 pm on April 15, 2025: member

   ACK 9ccee9cd022dcce72608831d4b1b93991345b306, changes look good.
7. TheCharlatan approved
8. 
   TheCharlatan commented at 12:36 pm on April 16, 2025: contributor

   ACK 9ccee9cd022dcce72608831d4b1b93991345b306
9. 
   hebasto commented at 12:48 pm on April 16, 2025: member

   cc @theuni

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ryanofsky DrahtBot hebasto TheCharlatan

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