Rewrite docs, focusing on the problem statement

[timholy]

Long-overdue, sorry.

This is a "test commit" for a single method. Let me know if you like this and what needs to change. If/when we get to a point where we are happy with the changes, I'll apply similar changes to the other docstrings in the package.

When finished this will fix #1046

[Copilot]

Pull request overview

Updates the trimr documentation to better describe the underlying linear system and how preconditioning/warm-starting behave, as a test run before applying similar edits across the package.

Changes:

• Expanded the problem statement for TriMR, including clearer notation and system description.
• Added detailed explanation of how M/N and ldiv define preconditioner application.
• Reorganized/rewrapped the docstring sections (interface, arguments, stopping criteria, references) for readability.

Comments suppressed due to low confidence (1)

src/trimr.jl:123

• Minor grammar: “for Hermitian and positive-definite linear system” should read “for a Hermitian positive-definite linear system” (and consider aligning singular/plural with the other bullets).

* `spd`: if `true`, set `τ = 1` and `ν = 1` for Hermitian and positive-definite
  linear system;

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[Copilot]

The docstring says TriMR solves a “symmetric” linear system, but the formulation uses Aᴴ (conjugate transpose) and later refers to a “partitioned Hermitian linear system”. Consider changing this wording to “Hermitian (symmetric if real)” to be accurate and internally consistent.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.69%. Comparing base (9536ef7) to head (c89736b).
⚠️ Report is 120 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1073      +/-   ##
==========================================
+ Coverage   94.68%   97.69%   +3.01%     
==========================================
  Files          45       50       +5     
  Lines        8027    10011    +1984     
==========================================
+ Hits         7600     9780    +2180     
+ Misses        427      231     -196     

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