[sysvabi64] document requirement for bti c in more detail

[nsz-arm]

the text currently has

"An executable or shared library that supports BTI must have a bti c instruction at the start of any entry that might be called indirectly."

but it's not clear if compilers should consider potential linker inserted veneers with indirect call/jump or if the linker should ensure that when a veneer is inserted it does not break bti compatibility.

(gcc+ld.bfd made different choice than llvm+lld)

see discussion at https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106671