update docs

Author: lkdvos

docs/src/user_interface/truncations.md

@@ -8,7 +8,18 @@ CollapsedDocStrings = true
 Currently, truncations are supported through the following different methods:
 
 ```@docs; canonical=false
+notrunc
 truncrank
 trunctol
 truncabove
+truncerror
+```
+
+It is additionally possible to combine truncation strategies by making use of the `&` operator.
+For example, truncating to a maximal dimension `10`, and discarding all values below `1e-6` would be achieved by:
+
+```julia
+maxdim = 10
+tol = 1e-6
+combined_trunc = truncrank(maxdim) & trunctol(tol)
 ```