luamod.lua: remove 'reference' and 'trigger'

Summary

As long as there are no plans to implement 'reference' and 'trigger' permissions, let's remove them from luamod.lua. This applies to grant/revoke privilege API functions.

Related issue in docs.

Related MR in docs.

src/luamod.lua

@@ -593,8 +593,6 @@ local supported_priveleges = {
     create = true,
     drop = true,
     alter = true,
-    reference = true,
-    trigger = true,
     insert = true,
     update = true,
     delete = true,
@@ -769,7 +767,7 @@ Params:
 
     2. privilege (string), one of
         'read' | 'write' | 'execute' | 'session' | 'usage' | 'create' | 'drop' |
-        'alter' | 'reference' | 'trigger' | 'insert' | 'update' | 'delete'
+        'alter' | 'insert' | 'update' | 'delete'
 
     3. object_type (string), one of
         'universe' | 'table' | 'sequence' | 'function' | 'role' | 'user'
@@ -788,6 +786,10 @@ Returns:
 
 Examples:
 
+    -- Enable the newly created user 'Dave' to perform essential actions,
+    -- eg. create a table, write to it, read data from it.
+    pico.grant_privilege('Dave', 'create', 'table')
+
     -- Grant read access to table 'Fruit' for user 'Dave'.
     pico.grant_privilege('Dave', 'read', 'table', 'Fruit')
 
@@ -802,6 +804,10 @@ Examples:
 
     -- Assign role 'Maintainer' to user 'Dave'.
     pico.grant_privilege('Dave', 'execute', 'role', 'Maintainer')
+
+Notice:
+
+    Only database admin can grant global privileges.
 ]]
 function pico.grant_privilege(grantee, privilege, object_type, object_name, opts)
     local ok, err = pcall(function()
@@ -894,7 +900,7 @@ Params:
 
     2. privilege (string), one of
         'read' | 'write' | 'execute' | 'session' | 'usage' | 'create' | 'drop' |
-        'alter' | 'reference' | 'trigger' | 'insert' | 'update' | 'delete'
+        'alter' | 'insert' | 'update' | 'delete'
 
     3. object_type (string), one of
         'universe' | 'table' | 'sequence' | 'function' | 'role' | 'user'