sh(1): Mention portability issue with shifting zero positional parameters.

Per Austin Group issue #459, shifting zero positional parameters may or may
not be considered an operand error (which causes the shell to exit in most
cases).


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274254 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f