Document scatter & scatter_if's preconditions.

Fixes #308
Document thrust::malloc.
Document thrust::free.
Puth thrust::malloc and thrust::free in the same documentation groups as device_malloc and device_free, respectively.
Document get_temporary_buffer & return_temporary_buffer.
Fix error in gather_if's documentation

Reported by Björn Hegstam
Document the policy-based versions of for_each & for_each_n.
Document the policy-based versions of thrust::adjacent_difference
Document the policy-based versions of thrust::copy, thrust::copy_n, and
thrust::copy_if
Document the policy-based versions of thrust::count and thrust::count_if.
Fix some minor documentation problems.
Document policy-based thrust::equal.
Document policy-based extrema functions.
Document policy-based thrust::fill and thrust::fill_n.
Document policy-based thrust::find, thrust::find_if, and thrust::find_if_not.
Document policy-based thrust::gather and thrust::gather_if.
Document policy-based thrust::generate and thrust::generate_n.
Document policy-based thrust::inner_product.
Document policy-based thrust::all_of, thrust::any_of, and thrust::none_of.
Document policy-based thrust::merge and thrust::merge_by_key.
Document policy-based thrust::mismatch.