Polishing: add comments.

sql/parse_file.cc

@@ -726,7 +726,11 @@ nlist_err:
                         TABLE)
     mem_root            MEM_ROOT for parameters allocation
     parameters          parameters description
-    required            number of required parameters in above list
+    required            number of parameters in the above list. If the file
+                        contains more parameters than "required", they will
+                        be ignored. If the file contains less parameters
+                        then "required", non-existing parameters will
+                        remain their values.
     hook                hook called for unknown keys
     hook_data           some data specific for the hook
 
@@ -909,6 +913,13 @@ list_err:
       }
     }
   }
+
+  /*
+    NOTE: if we read less than "required" parameters, it is still Ok.
+    Probably, we've just read the file of the previous version, which
+    contains less parameters.
+  */
+
   DBUG_RETURN(FALSE);
 }