genericirq.rst: add cross-reference links and use monospaced fonts

The document describes several functions that are documented
there via kernel doc macros. Add cross-references to them.

In order to be consistend with other documents, use monospaced
fonts for fields.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>