agent: Move gtk-doc comments from agent.c to agent.h for new API

agent: Move gtk-doc comments from agent.c to agent.h for new API

I completely disagree with this, and believe the C file is a much better
place for them, as then they’re:
 • easier to read while hacking on the functions, and
 • easier to modify once finished hacking on the functions.

I think the argument for putting them in the header files (so that the
documentation is available by the function declarations) is weak, as the
generated gtk-doc manual should be installed on the system alongside the
header files in any case.