[WMS-Server] Update WMS Server HOWTO with respect to layer.projection requirement

[dmorissette]

With the fix of bug 947 (and bug 862), it is no longer required to explicitly
provide a projection for every layer in a WMS mapfile (this is specific to WMS).
Layers with no projection are assumed to be in the same projection as the
top-level MAP projection and will automatically receive a copy of the map's
original projection if a new SRS is specified in a GetMap request.

The following paragraph from the WMS Server HOWTO should be edited to reflect that:

"However, if your server wants to advertise multiple projections, then at least
 a PROJECTION object is required in every layer, otherwise the layers won't be
 reprojected. This is just the way on-the-fly reprojection works in MapServer."

I think the new text should say that it is recommended to provide projections
for all layers to avoid possible errors, but that if none is provided then the
top-level map projecion will be assumed.

[jmckenna@…]

wms-server doc: removed old projection comment and added one mentioning the use
of the layer projection. committed to cvs.