Improve QLDocs of CallNode and MethodCallNode

When a function is assigned to a variable and called through that
variable then we can't always tell it was a method.

go/ql/lib/semmle/go/dataflow/internal/DataFlowNodes.qll

@@ -480,7 +480,11 @@ module Public {
   class CallNode extends ExprNode {
     override CallExpr expr;
 
-    /** Gets the declared target of this call */
+    /**
+     * Gets the declared target of this call, if it exists.
+     *
+     * This doesn't exist when a function is called via a variable.
+     */
     Function getTarget() { result = expr.getTarget() }
 
     private DataFlow::Node getACalleeSource() { result = getACalleeSource(this) }
@@ -637,14 +641,41 @@ module Public {
     /** Gets a result of this call. */
     Node getAResult() { result = this.getResult(_) }
 
-    /** Gets the data flow node corresponding to the receiver of this call, if any. */
+    /**
+     * Gets the data flow node corresponding to the receiver of this call, if any.
+     *
+     * When a method value is assigned to a variable then when it is called it
+     * looks like a function call, as in the following example.
+     *
+     * ```go
+     * file, _ := os.Open("test.txt")
+     * f := file.Close
+     * f()
+     * ```
+     *
+     * In this case we use local flow to try to find the receiver (`file` in
+     * the  above example).
+     */
     Node getReceiver() { result = this.getACalleeSource().(MethodReadNode).getReceiver() }
 
     /** Holds if this call has an ellipsis after its last argument. */
     predicate hasEllipsis() { expr.hasEllipsis() }
   }
 
-  /** A data flow node that represents a call to a method. */
+  /**
+   * A data flow node that represents a direct call to a method.
+   *
+   * When a method value is assigned to a variable then when it is called it
+   * syntactically looks like a function call, as in the following example.
+   *
+   * ```go
+   * file, _ := os.Open("test.txt")
+   * f := file.Close
+   * f()
+   * ```
+   *
+   * In this case it will not be considered a `MethodCallNode`.
+   */
   class MethodCallNode extends CallNode {
     MethodCallNode() { expr.getTarget() instanceof Method }