alloca.3: Prevent any misunderstanding about when allocated memory is released

alloca.3: Prevent any misunderstanding about when allocated memory is released

Perhaps some people might misunderstand memory allocated by
alloca() to be like other memory allocated on the stack: that when
the allocation (or the pointer to the allocation) goes out of
scope, the memory is freed. Add some text to prevent that
misunderstanding.

Reported-by: Robin Kuzmin <kuzmin.robin@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>