manual: add explanation that inlined functions are compiled with optimization settings of the call site

Inlined functions are compiled relatively to the optimizations settings on the call
site rather than definition site as one might expect.

  (declaim (inline foo))
  (defun foo (x y)
    (+ x y))

  (defun bar (a b)
    (declare (optimize speed))
    (foo a b))

JSnell explained that this is done deliberately so because it's just too damn convenient
in most cases.

I think the behaviour warrants explicit documentation in the manual, though.