Use FieldFilteredMarkup instead of calling XSS directly

Problem/Motivation

Currently the descriptions are being fed through the default XSS filter for security. That works, however it makes the descriptions provided by the module less flexible then the core field descriptions, because core has their own list of allowed tags.

Proposed resolution

Replace XSS references with FieldFilteredMarkup, which is the class the core field descriptions use to filter their markup. This way the module always stays in sync with what core would allow.