Interesting... I'm not familiar with sorrel, so I had to go look it up. Turns out they are nearly identical plants in the same genus Rumex. The commonest dock where I am is curly dock which is Rumex crispus. Common sorrel is Rumex acetosa.
Here's from wiki:
Common sorrel or garden sorrel (Rumex acetosa), often simply called sorrel, is a perennial herb that is cultivated as a garden herb or leaf vegetable (pot herb). Other names for sorrel include spinach dock and narrow-leaved dock.
"Narrow-leaved sorrel" and variants redirect here. These terms may also refer to curled dock (R. crispus)
So definitely confirms what I said -- they are all in the buckwheat family and edible. Your potato and sorrel soup is made with the sorrel leaves?
I just made for dinner last night a chinese cabbage and potato soup (to which I added black beans to make it substantial enough to be dinner on its own). I imagine it is a similar idea. When the dock pops up again, I will have to try this. I am working on doing more dealing with my weeds by eating them.
Unfortunately the version I usually have is the unrelated burdock, in the aster family, not the buckwheats. It is also used medicinally and is edible, but not as easily:
Edible parts: First-year roots and second-year stems can be cooked by boiling for about 20 minutes, then season to taste. Before cooking however, the stems should be peeled, and roots scrubbed in order to remove the bitter rind. Immature flower stalks may also be harvested in late spring, before flowers appear; their taste resembles that of artichoke. The Japanese have been known to eat the leaves when a plant is young and leaves are soft.