Consolidate guidance around examples (3.1.1) (#3893)

The four ways of specifying parameter or media type examples
are confusing. There is a lot of guidance that is repeated,
making it harder to see the essentials.  Some of that guidance is
also contradictory, with most saying that the examples SHOULD
match various conditions, but one saying that they MUST.

Research shows that SHOULD was repeatedly advocated, and instances
of MUST corrected back to SHOULD, except for one final commit
where MUST was introduced, and the subsequent commit to revert
to SHOULD seems to have simply missed one instance.  So this
change takes the position that the MUST was an error and weakens
it to SHOULD to match the other four or five places where the
requirement was specified.