build: Fix typo in sample/Makefile.am
cairo: Fix remaining leaks when finalizer gets called

Apply same finalizer leak fix to the rest of IDisposable-ownership
cairo classes as the one recently committed for Cairo.Context[1].

[1] https://github.com/mono/gtk-sharp/commit/41eeecbf9a6d8
doc: Run the automatic documentation updater
sample: Fix leak in Scribble sample when resizing the window