Clean up documentation on introspection rules conditions

A follow-up to commit d6ff54faf7.

Change-Id: Iff1a9b342b9c34debff5465e6f27500a20809f54

doc/source/usage.rst

@@ -61,11 +61,15 @@ Conditions
 
 A condition is represented by an object with fields:
 
-``op`` the type of comparison operation, default available operators include :
-``eq``, ``le``, ``ge``, ``ne``, ``lt``, ``gt`` (basic comparison operators),
-``in-net`` (checks that IP address is in a given network), ``contains``,
-``matches`` (check field against a regular expression, matches require a full
-match).
+``op`` the type of comparison operation, default available operators include:
+
+* ``eq``, ``le``, ``ge``, ``ne``, ``lt``, ``gt`` - basic comparison operators;
+
+* ``in-net`` - checks that an IP address is in a given network;
+
+* ``matches`` - requires a full match against a given regular expression;
+
+* ``contains`` - requires a value to contain a given regular expression.
 
 ``field`` a `JSON path <http://goessner.net/articles/JsonPath/>`_ to the field
 in the introspection data to use in comparison.