Fix example in README and docstring around how width actually works

Fixes #25

README.md

@@ -13,7 +13,7 @@ birth).  Every day it will choose a new memento randomly and put it into
 `global-mode-string' which is usually displayed in the mode line.  There is also
 an option to show it in the frame title instead of the global-mode-string.
 
-# Installation
+## Installation
 
 Install from [MELPA](https://melpa.org/#/memento-mori).
 
@@ -27,7 +27,7 @@ e.g. If you have something like the following in your emacs config
 ```emacs-lisp
 (require 'memento-mori)
 (setq memento-mori-mementos
-      '(("%.2f years old" :since "1970-01-01")))
+      '(("%.5Y years old" :since "1970-01-01")))
 (memento-mori-mode)
 ```
 
@@ -37,7 +37,7 @@ Or using `use-package`:
 (use-package memento-mori
   :ensure t
   :custom (memento-mori-mementos
-           '(("%.2f years old" :since "1970-01-01")))
+           '(("%.5Y years old" :since "1970-01-01")))
   :config (memento-mori-mode))
 ```
 

memento-mori.el

@@ -145,12 +145,15 @@ sequences below.  They also support all the flags supported by
 %F  The result of the `:formula' as a float.
 %%  A literal percent sign.
 
-As shown, most sequences have a upper case and a lower case
-version.  The lower case versions are truncated to integers since
-this is a common use case.  The upper case versions return a
-float for full control, but will likely require a width or
-precision specifier in practice, such as \"%.2Y\" to show the
-number of years with two decimal points of precision."
+As shown, most sequences have a upper case and a lower case version.
+The lower case versions are truncated to integers since this is a common
+use case.  The upper case versions return a float for full control, but
+will likely require a width or precision specifier in practice.
+Unfortunately, the width and precision specifiers behave like the
+corresponding ones in format when applied to %s, so they define the
+width in characters, not the number of decimal places.  Therefore,
+\"%.5Y\" will show the number of years with two or three decimal points
+of precision depending on if it's longer than 10 years or not."
   :group 'memento-mori
   :type '(repeat (cons
                   (string :tag "Format string")