Document the possibility of 2-letter XS types

[keckler]

There is an undocumented feature in ARMI:

armi/armi/reactor/blocks.py

Lines 406 to 411 in cfc2335

	Notes
	-----
	The single-letter use for xsType and buGroup limit users to 26 groups of each.
	ARMI will allow 2-letter xsType designations if and only if the `buGroups`
	setting has length 1 (i.e. no burnup groups are defined). This is useful for
	high-fidelity XS modeling of V&V models such as the ZPPRs.

It would be good to write this in the user documentation, maybe in this section about assembly blueprints.

Other related docstrings:

armi/armi/physics/neutronics/crossSectionGroupManager.py

Lines 1009 to 1013 in cfc2335

	"""
	Update the burnup group of each block based on its burnup.
	If only one burnup group exists, then this is skipped so as to accomodate the possibility
	of 2-character xsGroup values (useful for detailed V&V models w/o depletion).

armi/armi/physics/neutronics/crossSectionGroupManager.py

Lines 86 to 104 in cfc2335

	def getXSTypeNumberFromLabel(xsTypeLabel: str) -> int:
	"""
	Convert a XSID label (e.g. 'AA') to an integer.
	Useful for visualizing XS type in XTVIEW.
	2-digit labels are supported when there is only one burnup group.
	"""
	return int("".join(["{:02d}".format(ord(si)) for si in xsTypeLabel]))
	def getXSTypeLabelFromNumber(xsTypeNumber: int) -> int:
	"""
	Convert a XSID label (e.g. 65) to an XS label (e.g. 'A').
	Useful for visualizing XS type in XTVIEW.
	2-digit labels are supported when there is only one burnup group.
	"""