Added a lot more cases for enclosing strings with quotes

Author: javiereguiluz

components/yaml/yaml_format.rst

@@ -117,6 +117,19 @@ escaped with double quotes:
 * ``\L``
 * ``\P``
 
+Finally, there are other cases when the strings must be quoted, no matter if
+using single or double quotes:
+
+* when the string is ``true`` or ``false`` (otherwise, it would be treated as a
+  boolean value);
+* when the string is ``null`` or ``~`` (otherwise, it would be considered as a
+  ``null`` value);
+* when the string looks like a number, such as integers (e.g. ``2``, ``14``, etc.),
+  floats (e.g. ``2.6``, ``14.9``) and exponential numbers (e.g. ``12e7``, etc.)
+  (otherwise, it would be treated as a numeric value);
+* when the string looks like a date (e.g. ``2014-12-31``) (otherwise it would be
+  automatically converted into a Unix timestamp).
+
 When a string contains line breaks, you can use the literal style, indicated
 by the pipe (``|``), to indicate that the string will span several lines. In
 literals, newlines are preserved: