I'm pretty sure the watermarks they refer to in the help section are digital things that are part of the file and not actual marks on the picture, so any information on watermarks probably won't be of much use to you.
Removing watermarks generally means repainting the picture in some way or another. Sometimes it's as easy as selecting the watermarked area (using the lasso tool - I usually set it to polygonal lasso tool so I can more easy control the area I select) and changing the brightness and contrast. Normally I like to select the area and paste it in a new layer first so if I mess up I can delete the layer instead of having to restart the entire picture. Seperate layers are also good as you can use a soft-edged eraser to erase the edges of your selection and better blend it with the layer below.
The most loved tools for repairing images are probably the heal and clone tools (the little icons that look like a stamp and a bandaid, respectively). Choose one of these, hold down the alt key and click your mouse on an area of the image to copy that's similar in color and texture to the watermarked area, then paint over the area with the watermark. The clone tool will copy the selected area exactly, whereas the heal tool will attempt to match the brightness and color of selected area will the area being painted over.
Another trick I'm fond of is copying multiple areas of the picture and pasting them over the watermarked area. As I'm doing this, I'll flip and rotate pieces, erase areas to make them blend together, making a seamless collage to cover the watermark without resorting to pasting the same thing over and over again (which sometimes results in an obvious pattern that's almost as annoying as the original watermark).
Umm... those are just a few of the things I do. I'm not sure how familiar you are with Photoshop and, if necessary, I'd be glad to post a couple screen shots and images-in-progress. I'm not the best at explaining these things, but I'll try!