some more docs for constructing tensors from data

Author: Jutho

docs/src/man/tensors.md

@@ -333,6 +333,22 @@ for (c,b) in blocks(t3)
     println()
 end
 ```
+To construct the proper `data`, one has to know where to find each sector in the range
+`1:dim(V)` of every index `i` with associated space `V`, as well as the internal structure
+of the representation space when the corresponding sector `c` has `dim(c)>1`, i.e. in the
+case of `FusionStyle(c) isa NonAbelian`. Currently, the only non-abelian sectors are
+`Irrep[SU₂]` and `Irrep[CU₁]`, for which the internal structure is the natural one.
+
+There are some tools available to facilate finding the proper range of sector `c` in space
+`V`, namely `axes(V, c)`. This also works on a `ProductSpace`, with a tuple of sectors. An
+example
+```julia
+V = SU2Space(0=>3, 1=>2, 2=>1)
+P = V ⊗ V ⊗ V
+axes(P, (SU2Irrep(1), SU2Irrep(0), SU2Irrep(2)))
+```
+Note that the length of the range is the degeneracy dimension of that sector, times the
+dimension of the internal representation space, i.e. the quantum dimension of that sector.
 
 ### Constructing similar tensors