[json] document fact that key ordering is preserved (#10020)

lib/pure/json.nim

@@ -134,6 +134,13 @@
 ##    j2["details"] = %* {"age":35, "pi":3.1415}
 ##    echo j2
 
+runnableExamples:
+  ## Note: for JObject, key ordering is preserved, unlike in some languages,
+  ## this is convenient for some use cases. Example:
+  type Foo = object
+    a1, a2, a0, a3, a4: int
+  doAssert $(%* Foo()) == """{"a1":0,"a2":0,"a0":0,"a3":0,"a4":0}"""
+
 import
   hashes, tables, strutils, lexbase, streams, unicode, macros, parsejson