Function to call to get the next buffer to search.
When this variable is set to a function that returns a buffer, then
after typing another C-s or C-r at a failing search, the search goes
to the next buffer in the series and continues searching for the
next occurrence.
This function should return the next buffer (it doesn't need to switch
to it), or nil if it can't find the next buffer (when it reaches the
end of the search space).
The first argument of this function is the current buffer where the
search is currently searching. It defines the base buffer relative to
which this function should find the next buffer. When the isearch
direction is backward (when `isearch-forward' is nil), this function
should return the previous buffer to search.
If the second argument of this function WRAP is non-nil, then it
should return the first buffer in the series; and for the backward
search, it should return the last buffer in the series.