docs: Document high-level UTF-8 requirements for GLib

I’ve finally found the right place in the docs to put this stuff.

This doesn’t auto-link this section from every string in the GLib documentation, but I think that at this point (with gtk-doc in maintenance mode, and gi-docgen not fully applied to GLib) I don’t think we can do any better. The perfect is the enemy of the good, and having this stuff documented somewhere means that someone can link to it from multiple places in future somehow.

Signed-off-by: Philip Withnall pwithnall@endlessos.org

Fixes: #116 (closed)

Closes #116 (closed)