STR-782: add a warning about partial MuSig2 verification

[AaronFeickert]

Description

This PR updates the documentation for partial MuSig2 signature verification to caution against assuming unforgeability of partial signatures.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature/Enhancement (non-breaking change which adds functionality or enhances an existing one)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactor
• New or updated tests
• Dependency Update

Notes to Reviewers

Checklist

• I have performed a self-review of my code.
• I have commented my code where necessary.
• I have updated the documentation if needed.
• My changes do not introduce new warnings.
• I have added tests that prove my changes are effective or that my feature works.
• New and existing tests pass with my changes.

Related Issues

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 57.29%. Comparing base (506fd5c) to head (afd48f8).
Report is 5 commits behind head on main.

@@            Coverage Diff             @@
##             main     #558      +/-   ##
==========================================
- Coverage   57.33%   57.29%   -0.05%     
==========================================
  Files         309      309              
  Lines       31510    31510              
==========================================
- Hits        18067    18053      -14     
- Misses      13443    13457      +14     

Files with missing lines	Coverage Δ
crates/bridge-sig-manager/src/operations.rs	96.70% <ø> (ø)

... and 4 files with indirect coverage changes

[storopoli]

ACK afd48f8