Updating documentation

Author: jolyonb

docs/grading_math/sampling.md

@@ -242,7 +242,7 @@ Sample from any discrete set of values that are specified in a tuple. A single v
 
 ### DependentSampler
 
-Compute a value for a variable based on the values of other constants/variables. The sampler is simply initialized with the desired formula, which can use any base or user-defined functions (except for randomly sampled functions, see below). `DependentSampler`s can depend on other dependent variables. If you construct a self-referential chain, an error will occur. Note that `DependentSampler` can depend on vector/matrix quantities as well as scalars.
+Compute a value for a variable based on the values of other constants/variables. The sampler is simply initialized with the desired formula, which can use any base or user-defined functions (except for randomly sampled functions, see below). `DependentSampler`s can depend on other dependent variables. If you construct a self-referential chain, an error will occur. Note that `DependentSampler` can depend on vector/matrix quantities as well as scalars. You can even use sibling variables in a `DependentSampler`.
 
 ```pycon
 >>> # Set radius based on the random values of x, y and z
@@ -310,3 +310,6 @@ Finally, if you want to generate a complex random function, set `complex=True`.
 >>> functionsampler = RandomFunction(complex=True)
 
 ```
+
+!!! Note
+    - You cannot invoke a random functions within a `DependentSampler`. This is a limitation of the way that we have implemented random functions.