[Xcb] XCB documentation effort status update

[Michael Stapelberg]

Hi,

Status update: I "completed" documentation on 45 functions. With complete I
mean that there is some kind of explanation for all parameters, a description,
a list of possible errors and a 'see also' section. In some places, there are
still TODO entries for missing high-level or detail explanations. Help is
appreciated!

There are 4 requests left until I documented all requests which i3 uses. This
is the first milestone in this project and it should be reached by the end of
next week.

I updated the github repositories.

Best regards,
Michael