Convert parameter descriptions to Markdown for php:S1808

Since SQ 4.5, rule parameter descriptions are considered as Markdown.
This breaks compatibility with previous versions of SQ (since param
descriptions that make heavy use of Markdown will be close to unreadable
on versions < 4.5).
Ticket created: http://jira.codehaus.org/browse/SONARPHP-343.
Will be merged in next version.