Improve docstrings

Author: juliohm

src/simulation/field/fftsim.jl

@@ -24,8 +24,15 @@ accepts the following neighbor search options.
 
 ## References
 
-* Gutjahr 1997. [General joint conditional simulations using a fast
-  Fourier transform method](https://link.springer.com/article/10.1007/BF02769641)
+* Oliver 1995. [Moving averages for Gaussian simulation in two and three dimensions]
+  (https://link.springer.com/article/10.1007/BF02091660)
+
+* Ravalec et al. 2000. [The FFT Moving Average (FFT-MA) Generator: An Efficient
+  Numerical Method for Generating and Conditioning Gaussian Simulations]
+  (https://link.springer.com/article/10.1023/A:1007542406333)
+
+* Figueiredo et al. 2019. [Revisited Formulation and Applications of FFT Moving Average]
+  (https://link.springer.com/article/10.1007/s11004-019-09826-4)
 
 ### Notes