validation error when overriding imagecache preset

When overriding the imagecache preset with something like [l|files/image.jpg|text|preset=small] the preset is output as an attribute in the image tag, which breaks validation. This patch simply unsets $link['attributes']['preset'] before the attributes array is output.

I've also added in "text" in the example preset override (in the Default preset field's description text) since that placeholder wasn't obvious before when it was blank.