Fix bugs in implementation and documentation of the mapping API.

Fix bugs in implementation and documentation of the mapping API.

There were bugs in the mapping API, pointed to by Samuel Wales:

    Manual says:

     The following example counts the number of entries with TODO keyword
     WAITING, in all agenda files.

        (length (org-map-entries t "/+WAITING" nil 'agenda))

    Org says invalid function for the string.

First of all, the example in the manual was wrong, the "nil" should be
gone.  Second, the mapping function did not return all results in a
list, so that `length' could not count them.

This patch fixes these issues.