Remove references to sbrk to grow/shrink arenas

The manual incorrectly references sbrk as the method used to grow and
shrink heaps and the fact that M_TRIM_THRESHOLD and M_TOP_PAD control
that behavior.  In reality, a heap may be grown or shrunk through
multiple methods depending on whether it is the main arena (in which
case sbrk is correct) or not (in which case, there are a number of
strategies including allocating an additional heap to grow an arena
and/or 'mprotect' a region to make it available for allocation).

Remove references to sbrk so that it covers the behavior more
accurately.

	* manual/memory.texi (M_TOP_PAD): Remove reference to sbrk.
	(M_TRIM_THRESHOLD): Likewise.