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