docs(coercions): clarify LUB safety and target_feature rules

[PLeVasseur]

Summary

• Define safe/unsafe function signature subtyping and LUB selection to ensure mixed-safety branches resolve to unsafe fn pointers.
• Add target_feature coercion restriction for safe function pointers in non-enabled contexts.
• Closes [Change]: [1.93] LUB coercions now correctly handle function item types, and functions with differing safeties rust-lang/fls#649

Reference alignment

• https://doc.rust-lang.org/reference/type-coercions.html#least-upper-bound-coercions
• https://doc.rust-lang.org/reference/attributes/codegen.html#target_feature

Testing

• ./make.py