Document validation for CGI-based mapfile changes

[havatv]

From @sdlime in #26: There is a use case not covered in the docs that could (should?) be - setting validation for CGI-based mapfile changes (http://mapserver.org/cgi/controls.html#changing-map-file-parameters-via-a-form-or-a-url). At worst we should reference that content.

[sdlime]

@jmckenna, this functionality was removed in 8.0. So that section of controls.html should be removed from the documentation. Actually everything from that section on down should be removed. No need to keep anything related to version 5 and under, ROSA or Apache vars (that's in the config).

--Steve