Fix SpaceGroup.symbol to return short HM symbol always (incl. monoclinic spacegroups)

[janosh]

and update docs to say SpaceGroup.symbol gives short, not full HM symbols

question for @kaueltzen and @JaGeo: the SpaceGroup.symbol is documented to return the full HM symbol but as reported in spglib/moyo#83, it actually returns the short symbol for all but the 13 triclinic space groups. did you also notice this inconsistency? is there any reason for keeping that?
either way, @shyuep the docs need correcting and/or the attribute changed to actually return the full HM symbol

Originally posted by @janosh in #3845