Document remaining SciMLBase public API helpers

[ChrisRackauckas-Claude]

Summary

This is a focused follow-up to the SciMLBase public API documentation audit. It documents a set of SciMLBase-owned exported/public helper APIs at their source definitions and adds them to rendered interface docs.

Please ignore this PR until reviewed by @ChrisRackauckas.

Changes

• Added source docstrings and rendered @docs entries for public helper/stub APIs:
  • discretize, symbolic_discretize
  • clock namespace/domain APIs: Clocks, TimeDomain
  • integrator controls: get_dt, set_abstol!, set_reltol!, u_modified!, addsteps!
  • solution aliases/sentinels: AllObserved, SteadyStateSolution, QuadratureSolution
  • check_keywords
• Replaced the deprecated u_modified! macro form with an explicit deprecated method so the binding can carry a normal source docstring while preserving the deprecation warning behavior.
• Removed the redundant @doc attachment for AlgorithmInterpretation; the enum source docstring still resolves, and this removes the duplicate-doc precompile warning.

Audit notes

Audit method:

• Loaded SciMLBase in a temporary Julia environment.
• Enumerated public/exported names via names(SciMLBase; all=false) with Base.ispublic/Base.isexported.
• Cross-checked Docs.meta bindings for the fixed names and checked rendered @docs entries in docs/src.

Remaining audit candidates not handled in this focused PR are mostly dependency reexports/imported bindings (SciMLOperators.*, CommonSolve.solve/init/solve!) and a few broader SciMLBase constructor/function bindings that should be handled in separate follow-ups rather than expanding this PR into an omnibus docs rewrite.

Validation

• Pkg.test() passed locally with Julia +release.
• Runic.jl formatting applied to touched Julia files.
• git diff --check passed.
• docs/make.jl passed locally with Julia +release.

Known pre-existing docs warnings still emitted during docs build include undefined/manual warnings for AbstractQuadratureAlgorithm, several timeseries_steps_* ensemble entries, ModelingToolkit.PDESystem, and existing non-canonical docstrings. This PR did not add new failing docs warnings; docs build exits 0.