Update user docs for switch to POSIX semaphores.

Update user docs for switch to POSIX semaphores.

Since commit ecb0d20a9 hasn't crashed and burned, here's the promised
docs update for it.

In addition to explaining that Linux and FreeBSD ports now use POSIX
semaphores, I did some wordsmithing on pre-existing wording; in
particular trying to clarify which SysV parameters need to be set with
an eye to total usage across all applications.