1. Add short and long form string comparsion for sandesh levels

2. Add module documentation for derived stats.

Partial-Bug: #1618270
(cherry picked from commit 8d190a3)

Conflicts:
	library/common/derived_stats_results.sandesh

Change-Id: I280f7ef73021411c8156c09b8f94f7463b706bbd

compiler/generate/t_doc_generator.cc

@@ -627,25 +627,25 @@ t_doc_generator::sandesh_level::type t_doc_generator::string_to_sandesh_level(
   if (level == "debug") {
     return t_doc_generator::sandesh_level::DBG;
   }
-  if (level == "info") {
+  if (level == "informational" || level == "info") {
     return t_doc_generator::sandesh_level::INFO;
   }
   if (level == "notice") {
     return t_doc_generator::sandesh_level::NOTICE;
   }
-  if (level == "warn") {
+  if (level == "warning" || level == "warn") {
     return t_doc_generator::sandesh_level::WARN;
   }
-  if (level == "error") {
+  if (level == "error" || level == "err") {
     return t_doc_generator::sandesh_level::ERR;
   }
-  if (level == "critical") {
+  if (level == "critical" || level == "crit") {
     return t_doc_generator::sandesh_level::CRIT;
   }
   if (level == "alert") {
     return t_doc_generator::sandesh_level::ALERT;
   }
-  if (level == "emerg") {
+  if (level == "emergency" || level == "emerg") {
     return t_doc_generator::sandesh_level::EMERG;
   }
   return t_doc_generator::sandesh_level::INVALID;