Improve filter documentation

[1] http://maven.apache.org/plugins/maven-changes-plugin/jira-report-mojo.html#filter
and
[2] http://maven.apache.org/plugins/maven-changes-plugin/examples/customizing-jira-report.html#Using_JIRA_syntax

[1] says:

The filter parameter uses the same format of url parameters that is used in a JIRA search.

However, it fails to mention that the first & needs to be omitted.
And does not document what happens if used in conjunction with other forms of filter.

[2] compares the Jira filter with the Maven-style filter.

However, the Jira filter includes more than the Maven filter, as it can also be used to define sorting.

Indeed it appears that the Jira filter overrids all the Maven filters and the sort column names