cmp(1): Add EXAMPLES section

[fernape]

Summary

Add simple examples showing the use of -l, -z, stdin and offsets

Test Plan

• mandoc -Tlint clean
• igor clean
• aspell happy
• man ./cmp.1 renders the page properly

[bcr]

OK, you can commit that. Thanks!

head/usr.bin/cmp/cmp.1

 .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
 .\"     @(#)cmp.1  8.1 (Berkeley) 6/6/93
 .\" $FreeBSD$
 .\"
-.Dd May 1, 2018
+.Dd June 16, 2020
 .Dt CMP 1
 .Os
 .Sh NAME
 .Nm cmp
 .Nd compare two files
 .Sh SYNOPSIS
 .Nm
 .Op Fl l | s | x

 .Fl s
 option has not been specified,
 .Nm
 writes to standard error that EOF was reached in the shorter
 file (before any differences were found).
 .It >1
 An error occurred.
 .El
+.Sh EXAMPLES
+Assuming a file named example.txt with the following contents:
+.Bd -literal -offset indent
+a
+b
+c
+.Ed
+.Pp
+Compare stdin with example.txt:
+.Bd -literal -offset indent
+$ echo -e "a\\nb\\nc" | cmp - example.txt
+.Ed
+.Pp
+Same as above but introducing a change in byte three in stdin.
+Show the byte number (decimal) and differing byte (octal):
+.Bd -literal -offset indent
+$ echo -e "a\\nR\\nc" | cmp -l - example.txt
+     3 122 142
+.Ed
+.Pp
+Compare example.txt and /boot/loader.conf exiting if size differs.
+Note that
+.Fl z
+can only be used with regular files:
+.Bd -literal -offset indent
+$ cmp -z example.txt /boot/loader.conf
+example.txt /boot/loader.conf differ: size
+.Ed
+.Pp
+Compare stdin with file example.txt omitting the 4 first bytes from stdin and
+the 2 first bytes from example.txt:
+.Bd -literal -offset indent
+$ echo -e "a\\nR\\nb\\nc" | cmp - example.txt 4 2
+.Ed
 .Sh SEE ALSO
 .Xr diff 1 ,
 .Xr diff3 1
 .Sh STANDARDS
 The
 .Nm
 utility is expected to be
 .St -p1003.2