add a quick docstring to mspacegroup

Author: thchr

src/assembly/groups/mspacegroup.jl

@@ -1,3 +1,14 @@
+"""
+    mspacegroup(BNS₁, BNS₂)
+    mspacegroup(OG₃)        --> MSpaceGroup{3}
+
+Return the magnetic space group with BNS numbers `(BNS₁, BNS₂)` or the sequential OG number
+`OG₃` (from 1 to 1651).
+
+## Data sources
+The data underlying this function was retrieved from ISOTROPY's compilation of Daniel 
+Litvin's magnetic space group tables (http://www.bk.psu.edu/faculty/litvin/Download.html).
+"""
 function mspacegroup(BNS₁::Integer, BNS₂::Integer, Dᵛ::Val{D}=Val(3)) where D
     D == 3 || throw(DomainError("only 3D magnetic space groups are supported"))
     msgnum = (BNS₁, BNS₂)