bug#15797: 24.3.50; Info: Mention cache-long-scans

[Glenn Morris]

cd doc
find . -name '*.texi' -exec grep 'cache-long.*scans' {} +

./lispref/display.texi:@code{cache-long-scans} to @code{t}.
./lispref/display.texi:@defvar cache-long-scans
./lispref/positions.texi:performance of your code.  @xref{Truncation, 
cache-long-scans}.

So: cache-long-scans is defined, and there are no references to the old
name cache-long-line-scans.