docs: Elaborate replacement for g_strncasecmp

https://bugzilla.gnome.org/show_bug.cgi?id=568405
docs: Improve g_fopen description for Win32

https://bugzilla.gnome.org/show_bug.cgi?id=572466
docs: GSeekable improvements WIP
docs: Add missing parent interface descriptions
docs: Large update to gio-sections.txt

Hide lots of otherwise-unused symbols from the documentation.
docs: Fix typo G_SOCKET_ADDRESS_UNIX → G_UNIX_SOCKET_ADDRESS
docs: Small clean-up of howto subsection headers

This makes all the subsection headers consistent.

https://bugzilla.gnome.org/show_bug.cgi?id=659428
docs: Clarify how to unset a GFileInfo attribute

https://bugzilla.gnome.org/show_bug.cgi?id=592666
docs: Add simple GMarkup parser example

Based on an initial parser by Luc Pionchon.

https://bugzilla.gnome.org/show_bug.cgi?id=442929