Merge pull request #391 from thaJeztah/19.03_backport_session_endpoint_docs_updates

[19.03 backport] API: update docs that /session left experimental in V1.39
diff --git a/api/swagger.yaml b/api/swagger.yaml
index 0e021fe..d2dc488 100644
--- a/api/swagger.yaml
+++ b/api/swagger.yaml
@@ -10379,9 +10379,6 @@
       description: |
         Start a new interactive session with a server. Session allows server to call back to the client for advanced capabilities.
 
-        > **Note**: This endpoint is *experimental* and only available if the daemon is started with experimental
-        > features enabled. The specifications for this endpoint may still change in a future version of the API.
-
         ### Hijacking
 
         This endpoint hijacks the HTTP connection to HTTP2 transport that allows the client to expose gPRC services on that connection.
@@ -10415,4 +10412,4 @@
           description: "server error"
           schema:
             $ref: "#/definitions/ErrorResponse"
-      tags: ["Session (experimental)"]
+      tags: ["Session"]
diff --git a/docs/api/version-history.md b/docs/api/version-history.md
index c8327dd..2e5523e 100644
--- a/docs/api/version-history.md
+++ b/docs/api/version-history.md
@@ -87,6 +87,9 @@
 * `POST /swarm/init` now accepts a `DefaultAddrPool` property to set global scope default address pool
 * `POST /swarm/init` now accepts a `SubnetSize` property to set global scope networks by giving the
   length of the subnet masks for every such network
+* `POST /session` (added in [V1.31](#v131-api-changes) is no longer experimental.
+  This endpoint can be used to run interactive long-running protocols between the
+  client and the daemon.
 
 ## V1.38 API changes