sed: bring usage back into line with reality

[kevans]

Summary

These have been wrong in one way or another since BSD 4.4 Lite at a
minimum. BSD getopt(3) does not permute non-option arguments and we do
not do any option parsing after we hit the first one, so it makes no
sense to document any option as coming after the command.

While we're here, tweak the verbiage a little bit to point out the
difference between the two synopsis forms: that the presence of -e or
-f mean we will not interpret the first non-option argument as a
command. Also, consistently use 'command' in both the manpage and the
usage.

[pauamma_gundo.com]

Manual page changes LGTM with that typo fixed.

usr.bin/sed/sed.1

69

[des]

usr.bin/sed/main.c

201

why change “script” to the less accurate “command”?

[kevans]

usr.bin/sed/main.c

201

I just kind of picked one of the two ways we used to describe it. :-) I'll change them both to script.