SELFOUR-427: Correct discussion of untyped allocation
SELFOUR-427: Correct discussion of untyped allocation

The documentation for the untyped retype invocation in the manual
incorrectly stated the allocation policy. However, the correct
allocation policy was already described in the 'managing kernel memory'
section. This commit deletes the incorrect description and links
to the section where it is described correctly.

cmr/seL4,zhicheng/seL4,zhicheng/seL4,zhicheng/seL4,cmr/seL4,cmr/seL4