Bourne shell -> POSIX shell

The FreeBSD shell is a POSIX compatible shell. It evolved over several
decades from the Almquist shell, which was preceeded a decade before
that by the Bourne shell. Most readers today have never seen a Bourne
shell. If someone wants to learn to use our shell, they need to look for
tutorials on the POSIX shell. Align descriptions through out the tree
with this reality, consistent with it's manual and common parlance.

We made a similar change to the doc tree in b4d6eb01540fe.

MFC after: 3 days
Reviewed by: carlavilla
Differential Revision: https://reviews.freebsd.org/D56382