jsonpointer flatten/unflatten support [] and {}

Author: danielaparker

doc/ref/jsonpointer/flatten.md

@@ -11,9 +11,12 @@ Json unflatten(const Json& value, unflatten_options options = unflatten_options:
 ```
 
 (1) flattens a json object or array into a single depth object of JSON Pointer-value pairs.
-The keys in the flattened object are JSONPointer's.
-The values are primitive (string, number, boolean, or null). Empty objects or arrays become null. (until 0.160.0)
-The values are primitive (string, number, boolean, or null), empty object (`{}`) or empty array (`[]`). (since 0.160.0)
+
+- The keys in the flattened object are JSONPointer's.  
+
+- (until 0.160.0) The values are primitive (string, number, boolean, or null). Empty objects or arrays become null.   
+
+- (since 0.160.0) The values are primitive (string, number, boolean, or null), empty object (`{}`) or empty array (`[]`). 
 
 (2) unflattens a json object of JSON Pointer-value pairs. There is no unique solution,
 an integer appearing in a path could be an array index or it could be an object key.