Various distcheck and docs fixes

This fixes #1498 (closed), in the sense that I can no longer reproduce it. Given that the release rule in Makefile.am manually deletes the .deps directories, I suspect this problem is a deep-seated one which people have worked around for a long time. http://gnu-automake.7480.n7.nabble.com/make-distclean-td22165.html seems to do the same.

If #1498 (closed) reoccurs when I do the next 2.58.x release, I’ll a) get quite frustrated; b) potentially reopen it.

This also, mainly, is a load of docs fixes which don’t need backporting to glib-2-58. I got a bit carried away.

Closes #1498 (closed)