libthr(3): explain some internals of the locks implementation

libthr(3): explain some internals of the locks implementation

Describe internal allocations, mention problems with the use of global
malloc(3) and the reasons for internal allocator existence.

Document shared objects implementation and describe shortcomings of the
chosen approach, as well as the rationale why it was done that way.

Reviewed by:	markj
Discussed with:	jilles
Sponsored by:	The FreeBSD Foundation
MFC after:	3 days
Differential revision:	https://reviews.freebsd.org/D32243