DocBook has a rather detailed way of marking up descriptions of function behaviour. The tag that introduces it is <funcsynopsis>. Here is an example:
Example 7.9. Describing a function in a C library API
<funcsynopsis> <funcsynopsisinfo>#include <stdlib.h></funcsynopsisinfo> <funcprototype> <funcdef>double <function>atof</function></funcdef> <paramdef>const char *<parameter>nptr</parameter></paramdef> </funcprototype> </funcsynopsis>
Here is how it looks:
#include <stdlib.h>
double atof(nptr);
const char *nptr;