adding DEFRESOLUTION reference in documentation

[ochriste]

reporting RFC 55 DEFRESOLUTION parameter reference into the main Mapserver's mapfile documentation.

im not a native english speaker so maybe my phrasing will be a bit wonky.

by the way, I made some tests with a Mapserver 5.6.1 and I couldnt reproduce some of the element explained in the RFC 55:

for outlinewidth for line styles the behavior is exactly the same with and without size set: the outline scale.

for pixmap symbols the behavior is exactly the same with and without size set: the symbol scale.

[ochriste]

diff to map.txt with new DEFRESOLUTION documentation

[ochriste]

diff to label.txt & style.txt with new DEFRESOLUTION reference

[havatv]

MAP documentation has been updated (r11874). I have not included all the parameters that will be influenced by DEFRESOLUTION, since I was not sure that I would be able to cover everything. Consequently, the style document has not been updated.