Document LineSearch public APIs

[ChrisRackauckas-Claude]

This PR should be ignored until reviewed by @ChrisRackauckas.

Summary

• add source docstrings/examples for exported line-search algorithms and LineSearchSolution
• add missing exported APIs to the rendered native API page
• add a focused QA guard for exported API docstrings and docs-page entries
• remove the missing-docs warn-only setting now that exports are covered

Validation

• git diff --check
• timeout 3600 /home/crackauc/.juliaup/bin/julia +release --startup-file=no --project=. -e 'using Pkg; Pkg.instantiate(); Pkg.test()'
• timeout 3600 /home/crackauc/.juliaup/bin/julia +release --startup-file=no --project=test/qa -e 'using Pkg; Pkg.instantiate(); include("test/qa/qa.jl")'
• timeout 3600 /home/crackauc/.juliaup/bin/julia +release --startup-file=no --project=docs -e 'using Pkg; Pkg.develop(PackageSpec(path=pwd())); Pkg.instantiate(); include("docs/make.jl")'
• timeout 3600 /home/crackauc/.juliaup/bin/julia +release --startup-file=no --project=. -e 'using Pkg; Pkg.instantiate(); using Runic; exit(Runic.main(["--check", "."]))'