Improve some documentation

* Clarified the allowed values for "active-override" and
  "next-override", and make sure they recommend using the macro instead.
* Avoided misleading reference to "ido-everywhere" in README. Instead
  properly clarify the difference between it and this package.

Fxes #63.

README.md

@@ -1,12 +1,12 @@
 # ido-ubiquitous
 
-Gimme some ido... everywhere!
-
-Does what you were *really* hoping for when you did `(setq
-ido-everywhere t)`. Replaces stock emacs completion with ido
-completion wherever it is possible to do so without breaking things.
+Gimme some ido... everywhere! This pacakge replaces stock emacs
+completion with ido completion wherever it is possible to do so
+without breaking things.
 
 Get it from http://marmalade-repo.org/packages/ido-ubiquitous
 
 Note that ido-ubiquitous is not enabled for org mode or magit mode,
-because those modes have their own support for ido.
+because those modes have their own built-in support for ido, which you
+should enable instead. Also, to enable ido for all file and buffer
+completion, customize `ido-everywhere`.

ido-ubiquitous.el

@@ -470,14 +470,25 @@ this to non-nil, but this is not recommended."
 
 ;;; ido-ubiquitous core
 
+;; These variable are used to make ido-ubiquitous work properly in the
+;; case that `completing-read' is called recursively (which is
+;; possible when `enable-recursive-minibuffers' is non-nil.)
 (defvar ido-ubiquitous-next-call-replaces-completing-read nil
-  "If t, then the next call to `ido-completing-read' is by ido-ubiquitous.")
+  "If non-nil, then the next call to `ido-completing-read' is by ido-ubiquitous.")
 (defvar ido-ubiquitous-this-call-replaces-completing-read nil
-  "If t, then the current call to `ido-completing-read' is by ido-ubiquitous.")
+  "If non-nil, then the current call to `ido-completing-read' is by ido-ubiquitous.")
 (defvar ido-ubiquitous-next-override nil
-  "This holds the override to be applied on the next call to `completing-read'.")
+  "This holds the override to be applied on the next call to `completing-read'.
+
+It's value can be nil or one of the symbols `disable', `enable', or `enable-old'.
+
+You should not modify this variable directly. Instead use the macro `ido-ubiquitous-with-override'.")
 (defvar ido-ubiquitous-active-override nil
-  "This holds the override being applied to the current call to `completing-read'.")
+  "This holds the override being applied to the current call to `completing-read'.
+
+It's value can be nil or one of the symbols `disable', `enable', or `enable-old'.
+
+You should not modify this variable directly. Instead use the macro `ido-ubiquitous-with-override'.")
 
 (defun ido-ubiquitous-completing-read (&rest args)
   "Wrapper for `ido-completing-read' that enables ido-ubiquitous features.