Add more information to Commentary section

ido-ubiquitous.el

@@ -21,6 +21,26 @@
 ;; (almost) every function that uses the standard completion function
 ;; `completing-read'.
 
+;; To use this package, call `ido-ubiquitous-mode' to enable the mode,
+;; or use `M-x customize-variable ido-ubiquitous-mode' it to enable it
+;; permanently. Note that `ido-ubiquotous-mode' has no effect unless
+;; `ido-mode' is also enabled. Once the mode is enabled, most
+;; functions that use `completing-read' will now have ido completion.
+;; If you decide in the middle of a command that you would rather not
+;; use ido, just C-f or C-b at the end/beginning of the input to fall
+;; back to non-ido completion (this is the same shortcut as when using
+;; ido for buffers or files).
+
+;; Note that `completing-read' has some quirks and complex behavior
+;; that ido cannot emulate. Ido-ubiquitous attempts to detect some of
+;; these quirks and avoid using ido when it sees them. So some
+;; functions will not have ido completion even when this mode is
+;; enabled. Some other functions have ido disabled in them because
+;; their packages already provide support for ido via other means (for
+;; example, org-mode and magit). See `M-x customize-group
+;; ido-ubiquitous' and read about the override variables for more
+;; information.
+
 ;;; License:
 
 ;; This program is free software; you can redistribute it and/or modify