Add missing documentation.

Add documenation for functions require_kernel_sources, require_x86_64,
require_module, has_module, has_kernel_sources, get_cpu_stepping,
get_cpu_model, get_cpu_model_hex, get_number_cpus, cpu_model_min,
cpu_model_max, is_x86_64, and prepare_kernel_sources.
Merge pull request #6 from seanmalloy/doc-updates

Add missing documentation.