Man page update

[Perette Barella]

The man page for expr(1) should contain an exit status section.  The 
values for expr(1) are:

0 - result of expression is non-0 and non-NULL.
1 - result of expression is 0 or NULL.
2 - expression is invalid.

This addition is important because a valid expression can return a 
non-0 exit status, which is unusual.