Improve docstring for select_sif_problems: clarify max_con=0 vs contype="unc" and bounds

[arnavk23]

This PR improves the docstring of select_sif_problems to make it clearer and more user-friendly, by explaining subtle points that commonly cause confusion:

Clarifies what max_con=0 actually does:

• Keeps problems with no linear or nonlinear constraints,
• But still allows bound constraints (variable lower/upper bounds).

Explains what contype="unc" means:

• Keeps only truly unconstrained problems:
  • No linear constraints,
  • No nonlinear constraints,
  • And no variable bounds.

Adds an explicit note about variable bounds not being counted in max_con.

Provides a clear idiom to get truly unconstrained problems:

filter(is_truly_unconstrained, select_sif_problems())

Adds better structured explanation of:

• only_* flags
• objtype and contype filters
• custom_filter

Includes realistic examples covering typical use cases.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.23%. Comparing base (f2ad6a4) to head (0bae590).
Report is 153 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #457       +/-   ##
===========================================
- Coverage   89.11%   74.23%   -14.88%     
===========================================
  Files           5        7        +2     
  Lines         790     1669      +879     
===========================================
+ Hits          704     1239      +535     
- Misses         86      430      +344     

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[amontoison]

Very useful PR 👍👍
Thank you @arnavk23 !