control to the next back-end. The function should return `stop' if it
should complete but cannot (e.g. if it is in the middle of a string).
Instead of a string, the back-end may return a cons where car is the prefix
-and cdr is used in `company-minimum-prefix-length' test. It must be either
-number or t, and in the latter case the test automatically succeeds.
+and cdr is used instead of the actual prefix length in the comparison
+against `company-minimum-prefix-length'. It must be either number or t,
+and in the latter case the test automatically succeeds.
`candidates': The second argument is the prefix to be completed. The
return value should be a list of candidates that match the prefix.