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

[Tobias C. Rittweiler]

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.