Improve docs for SYMBOL GAP

[dmorissette]

The docs for the SYMBOL GAP parameter need to be updated to better document the way this parameter works. The current version isn't clear:

See http://mapserver.org/mapfile/symbol.html

There was also a change of behavior between v4.10 and 5.0 that should be taken into account, see the thread at ​http://lists.osgeo.org/pipermail/mapserver-dev/2009-June/008808.html

I'll attach an image that shows the 4.10 and 5.0 behaviors for positive and negative GAP values (what needs to be documented).

[dmorissette]

Image showing 4.10 and 5.0 behavior for positive and negative GAP values

[havatv]

Documentation has changed. If it still needs to be improved, please provide details and reopen.