Add EXAMPLES to cover all flags except -f since it is analogous to -s.

An EXAMPLE section was adding with some basic examples that show the use of
uniq(1) with various flags.

Submitted by: fernape@
Approved by: bcr@
MFC after: 4 days
Relnotes: yes (EXAMPLE section for uniq(1))
Differential Revision: https://reviews.freebsd.org/D25149