Minimal timer period of 100us introduced in r244758 is overkill.  While
original 2us are indeed not enough, 3us are working quite well on my tests.
To be more safe set minimal period to 5us and to be even more safe replicate
here from HPET mechanism of rereading counter after programming comparator.

This change allows to handle 30K of short nanosleep() calls per second on
Raspberry Pi instead of just 8K before.

Discussed with:	gonzo
Document uma_find_refcnt().
Document some flags to the uma_zcreate(). Not all flags are documented,
only those that at least are used in the kernel, or that definitely
work.