C#: Add Ql docs to synthetic fields.

csharp/ql/lib/semmle/code/csharp/dataflow/internal/DataFlowPrivate.qll

@@ -2040,9 +2040,11 @@ predicate allowParameterReturnInSelf(ParameterNode p) {
   FlowSummaryImpl::Private::summaryAllowParameterReturnInSelf(p)
 }
 
+/** A synthetic field. */
 abstract class SyntheticField extends string {
   bindingset[this]
   SyntheticField() { any() }
 
+  /** Gets the type of this synthetic field. */
   Type getType() { result instanceof ObjectType }
 }

csharp/ql/lib/semmle/code/csharp/dataflow/internal/DataFlowPublic.qll

@@ -224,11 +224,13 @@ class FieldContent extends Content, TFieldContent {
   deprecated override Gvn::GvnType getType() { result = Gvn::getGlobalValueNumber(f.getType()) }
 }
 
+/** A reference to a synthetic field. */
 class SyntheticFieldContent extends Content, TSyntheticFieldContent {
   private SyntheticField f;
 
   SyntheticFieldContent() { this = TSyntheticFieldContent(f) }
 
+  /** Gets the underlying synthetic field. */
   SyntheticField getField() { result = f }
 
   override string toString() { result = "synthetic " + f.toString() }