Add documentation for getDependentDialects() in the PassManagement infra docs

mlir/docs/PassManagement.md

struct MyOperationPass : public OperationPass<MyOperationPass> {
   void runOnOperation() override {
     // Get the current operation being operated on.
     Operation *op = getOperation();
     ...
   }
 };
 ```
 
+### Dependent Dialects
+
+Dialects must be loaded in the MLIRContext before entities from these dialects
+(operations, types, attributes, ...) can be created. Dialects must be loaded
+before starting the multi-threaded pass pipeline execution. To this end, a pass
+that can create an entity from a dialect that isn't already loaded must express
+this by overriding the `getDependentDialects()` method and declare this list of
+Dialects explicitly.
+
 ## Analysis Management
 
 An important concept, along with transformation passes, are analyses. These are
 conceptually similar to transformation passes, except that they compute
 information on a specific operation without modifying it. In MLIR, analyses are
 not passes but free-standing classes that are computed lazily on-demand and
 cached to avoid unnecessary recomputation. An analysis in MLIR must adhere to
 the following:

 It contains the following fields:
 
 *   summary
     -   A short one line summary of the pass, used as the description when
         registering the pass.
 *   description
     -   A longer, more detailed description of the pass. This is used when
         generating pass documentation.
+*   dependentDialects
+    -   A list of strings that are the Dialect classes this pass can introduce.
 *   constructor
     -   A piece of C++ code used to create a default instance of the pass.
 *   options
     -   A list of pass options used by the pass.
 *   statistics
     -   A list of pass statistics used by the pass.
 
 #### Options