clarify comments

And throw instead of return null for invalid settings.

include/json/reader.h

@@ -299,10 +299,17 @@ public:
     These are case-sensitive.
     Available settings (case-sensitive):
     - "collectComments": false or true
-    - "allowComments"
-    - "strictRoot"
-    - "allowDroppedNullPlaceholders"
-    - "allowNumericKeys"
+      - true to collect comment and allow writing them
+        back during serialization, false to discard comments.
+        This parameter is ignored if allowComments is false.
+    - "allowComments": false or true
+      - true if comments are allowed.
+    - "strictRoot": false or true
+      - true if root must be either an array or an object value
+    - "allowDroppedNullPlaceholders": false or true
+      - true if dropped null placeholders are allowed. (See StreamWriterBuilder.)
+    - "allowNumericKeys": false or true
+      - true if numeric object keys are allowed.
 
     You can examine 'settings_` yourself
     to see the defaults. You can also write and read them just like any

include/json/writer.h

@@ -92,9 +92,9 @@ public:
     Available settings (case-sensitive):
     - "commentStyle": "None" or "All"
     - "indentation":  "<anything>"
-    - "enableYAMLCompatibility": False or True
+    - "enableYAMLCompatibility": false or true
       - slightly change the whitespace around colons
-    - "dropNullPlaceholders": False or True
+    - "dropNullPlaceholders": false or true
       - Drop the "null" string from the writer's output for nullValues.
         Strictly speaking, this is not valid JSON. But when the output is being
         fed to a browser's Javascript, it makes for smaller output and the

src/lib_json/json_writer.cpp

@@ -981,7 +981,7 @@ StreamWriter* StreamWriterBuilder::newStreamWriter() const
   } else if (cs_str == "None") {
     cs = CommentStyle::None;
   } else {
-    return NULL;
+    throw std::runtime_error("commentStyle must be 'All' or 'None'");
   }
   std::string colonSymbol = " : ";
   if (eyc) {