encoding/json/jsontext, encoding/json/v2: document experimental nature

Change-Id: I7b2c391749e0113e006f37b2ac1ebfe3ee0a4e0e
Reviewed-on: https://go-review.googlesource.com/c/go/+/680715
TryBot-Bypass: Damien Neil <dneil@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Joseph Tsai <joetsai@digital-static.net>
Auto-Submit: Damien Neil <dneil@google.com>

src/encoding/json/jsontext/doc.go

@@ -10,6 +10,11 @@
 // primitive data types such as booleans, strings, and numbers,
 // in addition to structured data types such as objects and arrays.
 //
+// This package (encoding/json/jsontxt) is experimental,
+// and not subject to the Go 1 compatibility promise.
+// It only exists when building with the GOEXPERIMENT=jsonv2 environment variable set.
+// Most users should use [encoding/json].
+//
 // The [Encoder] and [Decoder] types are used to encode or decode
 // a stream of JSON tokens or values.
 //

src/encoding/json/v2/doc.go

@@ -9,6 +9,11 @@
 // primitive data types such as booleans, strings, and numbers,
 // in addition to structured data types such as objects and arrays.
 //
+// This package (encoding/json/v2) is experimental,
+// and not subject to the Go 1 compatibility promise.
+// It only exists when building with the GOEXPERIMENT=jsonv2 environment variable set.
+// Most users should use [encoding/json].
+//
 // [Marshal] and [Unmarshal] encode and decode Go values
 // to/from JSON text contained within a []byte.
 // [MarshalWrite] and [UnmarshalRead] operate on JSON text