"contains the list of files match in both directories," should have 
"that" before "match"

In addition I couldn't figure out from the documentation what "common" 
was supposed to be doing -- it sounded more like something that should 
be part of the result not a parameter. I had to look at the code to 
realize that this function ONLY compares the files listed in common -- 
that it doesn't compare two directories completely, but rather dir1 and 
dir2 are merely prefixes for the files listed in common that are to be 
compared.

One thing that would help would be to move the paragraph about common 
first. Another would be to "a list of file names found in both 
directories" to "a list of the names of files to be compared, which must 
be in both directories" or something like that.