Update and clean up the UMA man page.

• Fix warnings from igor and mandoc.
• Provide a brief description of the separation between zones and their backend slab allocators.
• Document cache zones and secondary zones.
• Document the kernel config options added in r350659.
• Document the uma_zalloc_pcpu() and uma_zfree_pcpu() wrappers.
• Document uma_zone_reserve(), uma_zone_reserve_kva() and uma_zone_prealloc().
• Document uma_zone_alloc() and uma_zone_freef().
• Add some missing MLINKs and Xrefs.

MFC after: 2 weeks