documentation: Fix some build warnings
When building gnome-todo, I see several warnings during the build: build_warnings.
This MR fixes some of them. There are still some warnings, but I don't know how to solve them:build_warnings_4 The documentation comments look correct to me where these warnings are. So, I don't know the cause of them there.
Also, there were some parameters missing in the documentation comments which are added with this MR, too.
Related to: #93 (The warnings in this issue are different, but it is the same area which is affected (
Generating Gtd-1.0.gir with a custom command.). Also this issue is pretty old, so some things may have changed.)