[WMS Server HOWTO] Fix ref toinvalid GetMap request

[dmorissette]

The WMS Server HOWTO contains a section titled "Test with a Real Client" that
contains an example of an incomplete GetMap request to use for testing:

http://mapserver.gis.umn.edu/doc/wms-server-howto.html#val_getmap

This used to work with V4.0 and before, but since 4.1, we do a stronger checking
on parameter values, and at least the LAYERS parameter is required. (Other
mandatory parameters should be checked but they are not.)

We have two options:
1- Modify the example to include LAYERS, and keep the note that states that this
not a valid GetMap request.  Also try to make the note stand out more since some
people seem to overlook it.
2- Modify this section to show how to build a complete GetMap request with all
the mandatory parameters included (VERSION, REQUEST, LAYERS, STYLES, SRS, BBOX,
WIDTH, HEIGHT and FORMAT)

I think #1 is simpler, but #2 would be more "correct".  Or actually, we could
have a combination of both: start with #1, and then explain that this is not a
valid request according to the spec and then explain how to build a valid request.

[jmckenna@…]

Added both a starting request and a valid request according to the spec (with
examples).  also cleaned up the doc for WMS 1.1.1.  committed to main doc CVS.