Document the new behavior regarding height-for-width, and specifically
add a recommendation to use GTK_SIZE_GROUP_BOTH if the widgets are meant
to have a common size in both orientations, even when the sizes being
equal in one of them in ensured by some different mechanism.

Mention and recommend other ways to achieve equal sizes between widgets.

Also update to more modern gi-docgen syntax, and do other small tweaks.

Signed-off-by: Sergey Bugaev <bugaevc@gmail.com>