Document why customize functions are where they are (#239)

Author: knopers8

Framework/src/runAdvanced.cxx

@@ -41,7 +41,9 @@ using namespace o2::framework;
 
 // Additional configuration of the topology, which is done by implementing `customize` functions and placing them
 // before `runDataProcessing.h` header. In this case, both Dispatcher and Merger are configured to accept incoming
-// messages without waiting for the rest of inputs.
+// messages without waiting for the rest of inputs. The `customize` functions have to be above
+// `#include "Framework/runDataProcessing.h"` - that header checks if these functions are defined by user and if so, it
+// invokes them. It uses a trick with SFINAE expressions to do that.
 void customize(std::vector<CompletionPolicy>& policies)
 {
   DataSampling::CustomizeInfrastructure(policies);

Framework/src/runBasic.cxx

@@ -39,6 +39,10 @@
 using namespace o2;
 using namespace o2::framework;
 
+// The customize() functions are used to declare the executable arguments and to specify custom completion and channel
+// configuration policies. They have to be above `#include "Framework/runDataProcessing.h"` - that header checks if
+// these functions are defined by user and if so, it invokes them. It uses a trick with SFINAE expressions to do that.
+
 void customize(std::vector<CompletionPolicy>& policies)
 {
   DataSampling::CustomizeInfrastructure(policies);

Framework/src/runMergerTest.cxx

@@ -22,6 +22,10 @@
 #include <random>
 using namespace o2::framework;
 
+// The customize() functions are used to declare the executable arguments and to specify custom completion and channel
+// configuration policies. They have to be above `#include "Framework/runDataProcessing.h"` - that header checks if
+// these functions are defined by user and if so, it invokes them. It uses a trick with SFINAE expressions to do that.
+
 void customize(std::vector<CompletionPolicy>& policies)
 {
   CompletionPolicy mergerConsumesASAP =

Framework/src/runQC.cxx

@@ -31,6 +31,10 @@
 using namespace o2;
 using namespace o2::framework;
 
+// The customize() functions are used to declare the executable arguments and to specify custom completion and channel
+// configuration policies. They have to be above `#include "Framework/runDataProcessing.h"` - that header checks if
+// these functions are defined by user and if so, it invokes them. It uses a trick with SFINAE expressions to do that.
+
 void customize(std::vector<ConfigParamSpec>& workflowOptions)
 {
   workflowOptions.push_back(