On some of my projects I noticed white lines on the top and bottom of the audio clips. I can't find anything about them in the documentation. Does someone know what they are, or what they mean?

Those are likely overload indicators. You clipped the signal.

Or it may be that they're linked clips, i.e., under Options > Drag and Drop Options "Copy Entire Clips as Linked Clips" is checked.

A picture tells a thousand words, BG. Can you post a screenshot?

White lines in the audio clips probably means the audio clips are having a real good time. :-)

possibly snap offsets

Who's taking bets on what the lines are? I'm with ampfixer - signals are clipped. But all are good suggestions...I was going to say transients? But those would be "a lot of lines.." or not.

I'm betting on linked clips. Or bumps.

Why do people waste our collective time by asking questions and never coming back? It would be nice if the reference manual was up-to-date, then we could just list a page number.

I had a chance to do some testing this evening, and the white horizontal lines were at the top and bottom of the selected audio clip.  The lines extend the full length of the clip.  On some clips the bottom line has a 45 degree angle at each end, and the angled ends do not appear to be related to groove clip looping.  I checked in preferences->colors and I could not find anything. 
Here's the image.  The clip on the right is selected.
 This one does not have the 45 degree angle on the bottom white line.