[clang] Note that optnone and target attributes do not apply to nested functions (#82815)

This behavior is true for all attributes, but this behavior can be
surprising for attributes which have function-wide effects, such as
`optnone` and `target`. Most other function attributes affect the
prototype or semantics, but do not affect code generation in the
function body. I believe it is worth calling this out in the
documentation of these function-wide attributes. There may be more,
these were the two that came to mind.