Outer limit on size of undo information for one command.
At garbage collection time, if the current command has produced
more than this much undo information, it discards the info and displays
a warning. This is a last-ditch limit to prevent memory overflow.
The size is counted as the number of bytes occupied, which includes
both saved text and other data. A value of nil means no limit. In
this case, accumulating one huge undo entry could make Emacs crash as
a result of memory overflow.
In fact, this calls the function which is the value of
`undo-outer-limit-function' with one argument, the size.
The text above describes the behavior of the function
that variable usually specifies.