doc: improvements to doc/descriptors.md

[sipa]

This brings doc/descriptors.md up to date:

• Stop trying to exhaustively list all RPCs that involve descriptors. They're used everywhere.
• Stop trying to give the history of descriptor support, we have release notes for that.
• Mention that wallets are now built around descriptors (especially with legacy wallets gone).
• Mention musig() KEY expressions in the specification part.
• Reference the relevant output descriptor BIPs in the text.

[DrahtBot]

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/33986.

Reviews

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Type	Reviewers
ACK	rkrux, w0xlt

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LLM Linter (✨ experimental)

Possible typos and grammar issues:

• P2SH embed the argument. -> P2SH embeds the argument. [subject-verb agreement; "P2SH" (singular) requires "embeds"]
• P2WSH embed the argument. -> P2WSH embeds the argument. [subject-verb agreement; "P2WSH" (singular) requires "embeds"]

^2025-12-01

[fanquake]

cc @Sjors

[rkrux]

lgtm ACK fc33626

[w0xlt]

ACK fc33626