improve the compiler option "cppCompileToNamespace", a custom namespace can now be set

doc/advopt.txt

@@ -81,7 +81,9 @@ Advanced options:
   --NimblePath:PATH         add a path for Nimble support
   --noNimblePath            deactivate the Nimble path
   --noCppExceptions         use default exception handling with C++ backend
-  --cppCompileToNamespace   use namespace "Nim" for the generated C++ code
+  --cppCompileToNamespace:namespace
+                            use the provided namespace for the generated C++ code,
+                            if no namespace is provided "Nim" will be used
   --excludePath:PATH        exclude a path from the list of search paths
   --dynlibOverride:SYMBOL   marks SYMBOL so that dynlib:SYMBOL
                             has no effect and can be statically linked instead;