Improve docs for -d.
As mentioned in this commit brings the docs up to date with the man page
and usage statement.
Fixes
@@ -57,9 +57,9 @@ Available options are:
Print only number of matches.
-.. option:: -d <identifier>=<value>
+.. option:: -d <identifier>=<value> --define=identifier=value
- Define external variable.
+ Define external variable. This option can be used multiple times.
.. option:: --fail-on-warnings
