Expand the libthr(3) manpage to document knobs accepted by libthr.so
and explain some internal working of the library, neccessary to
understand the knobs effects.

Reviewed by:	bjk, pluknet
Sponsored by:	The FreeBSD Foundation
MFC after:	3 weeks


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272070 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f