Update references for magnetic space group information.

[jamesrhester]

As per para (3) of update document from @brantonc :

(3) The descriptions of the following tags needed minor updates.
_space_group_magn.name_BNS
_space_group_magn.name_OG
_space_group_magn.number_BNS
_space_group_magn.point_group_number
Unnecessary cross references were omitted.
Spaces and enclosing quotes were removed from the examples.
The new Acta Cryst A paper on UNI symbols is referenced in some entries.
An additional example was added.

[jamesrhester]

Merging on the basis that the magcif authors have already approved these changes.