docs: clarify SIMD build requirements

[brndnmtthws]

Summary

Clarify that dryoc's portable SIMD backends require the simd_backend and nightly features, but do not require target-specific RUSTFLAGS.

User Impact

Users no longer receive unnecessary CPU-specific compiler flag recipes when enabling portable SIMD. Dependency-provided SIMD backends continue to select supported implementations independently.

Root Cause

The documentation retained older compile-time backend guidance after dryoc moved its SIMD implementations to portable SIMD and dependencies adopted their own backend selection.

Fix

• Replace the target-specific RUSTFLAGS list with explicit no-flags guidance.
• Remove the inaccurate build-time qualifier from the curve25519-dalek backend description.
• Keep the README and crate-level rustdoc synchronized.

Validation

• env -u RUSTFLAGS -u CARGO_ENCODED_RUSTFLAGS cargo +nightly test --features simd_backend,nightly
• RUSTDOCFLAGS='-D warnings' env -u RUSTFLAGS -u CARGO_ENCODED_RUSTFLAGS cargo +nightly doc --no-deps --features serde,wincode,nightly,simd_backend
• cargo +nightly fmt --all -- --check

[netlify]

✅ Deploy Preview for dryoc-docs ready!

Name	Link
🔨 Latest commit	95d6354
🔍 Latest deploy log	https://app.netlify.com/projects/dryoc-docs/deploys/6a58cbdb5988a00007d8d5fe
😎 Deploy Preview	https://deploy-preview-148--dryoc-docs.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify project configuration.

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.44%. Comparing base (dd9e59b) to head (95d6354).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #148      +/-   ##
==========================================
- Coverage   82.49%   82.44%   -0.05%     
==========================================
  Files          58       58              
  Lines        4427     4427              
==========================================
- Hits         3652     3650       -2     
- Misses        775      777       +2     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.