(Visiting): Document large-file-warning-threshold.

(Visiting): Document large-file-warning-threshold.
Move paragraph on file-selection dialog.
Mention visiting files using X drag and drop.
(Reverting): Mention using Auto-Revert mode to tail files.
Document auto-revert-tail-mode.
(Version Systems): Minor correction.
(Comparing Files): Diff-mode is no longer based on Compilation mode.
Document compare-ignore-whitespace.
(Misc File Ops): Explain passing a directory to rename-file.
Likewise for copy-file and make-symbolic-link.