docs: add description for inference

docs/source/inference/index.rst

@@ -6,11 +6,11 @@ This module hosts functionality for inference methods.
 
 Currently implemented classes are:
 
-* :class:`.ActiveLearning`:
+* :class:`.ActiveLearning`: Construct a Gaussian process emulator for an unnormalized posterior. 
 
-* :class:`.GridEstimation`:
+* :class:`.GridEstimation`: Estimate a posterior distribution by grid estimation.
 
-* :class:`.MetropolisHastingsEstimation`:
+* :class:`.MetropolisHastingsEstimation`: Estimate a posterior by Metropolis Hastings estimation.
 
 .. toctree::
    :maxdepth: 1