doc: mention -DWITH_ZMQ=ON in macOS build guide #35928

doc/build-osx.md currently says:

Support for ZMQ notifications requires the following dependency.

The zeromq dependency is covered, but the -DWITH_ZMQ=ON CMake option is not mentioned anywhere in that section. WITH_ZMQ defaults to OFF, so following the guide as written results in a build with ZMQ disabled, even though the user completed the ZMQ section. I verified this at the configure step on macOS: with zeromq installed and no flag, CMake reports ZeroMQ ... OFF; with -DWITH_ZMQ=ON it reports ON.

The same wording was added to the BSD build guides in #35283, but doc/build-osx.md was not included.

Docs-only change. No tests run.

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For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35928.

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ACK 5ee6695932a29f65cc638c61abf481c1e659fdd1.

Since we're modifying this, it might be a good time to unify the whole paragraph with the other BSD docs.

This follows hebasto's suggestion and unifies the paragraph with the BSD build guides.

The section structure in doc/build-osx.md was intentionally left unchanged. Unlike the BSD guides, doc/build-osx.md consistently groups dependencies under * Dependencies headings (Wallet, IPC, GUI, ZMQ, Test Suite, and Deploy). Renaming only the ZMQ section to match the BSD layout would make it inconsistent with the rest of the document, while restructuring the document as a whole would be outside the scope of this PR.

The Skip if you do not need ZMQ functionality. line was also left unchanged, as the document already contains a similar note in the preceding QR section.

Thanks! You can now squash all commits.

ACK 222855ed1129669e67785c9800292f017a301816.