layers: Change pNext validation error to warning

Change the severity level of the parameter_validation layer's pNext
parameter validation messages from error to warning.  Also add an
explanation that the warning is derived from the Valid Usage
language from version X of the Vulkan spec/header, which may be out
of date:

This warning is based on the Valid Usage documentation for version %d
of the Vulkan header.  It is possible that you are using a struct
from a private extension or an extension that was added to a later
version of the Vulkan header, in which case your use of pNext is
perfectly valid but is not guaranteed to work correctly with
validation enabled

Issues-Addressed: LunarXchange 560
Change-Id: I97f55cba7538506d246e45ef603a86db9ad1adcb
diff --git a/generator.py b/generator.py
index 26fe99e..65c6a40 100755
--- a/generator.py
+++ b/generator.py
@@ -3431,7 +3431,7 @@
             extStructCount = 'ARRAY_SIZE(allowedStructs)'
             extStructVar = 'allowedStructs'
             extStructNames = '"' + ', '.join(structs) + '"'
-        checkExpr.append('skipCall |= validate_struct_pnext(report_data, "{}", "{}", {}, {}{}, {}, {});\n'.format(
+        checkExpr.append('skipCall |= validate_struct_pnext(report_data, "{}", "{}", {}, {}{}, {}, {}, GeneratedHeaderVersion);\n'.format(
             funcPrintName, valuePrintName, extStructNames, prefix, value.name, extStructCount, extStructVar))
         return checkExpr
     #