API: update docs that /session left experimental in V1.39

The `/session` endpoint left experimental in API V1.39 through
239047c2d36706f2826b0a9bc115e0a08b1c3d27 and
01c9e7082eba71cbe60ce2e47acb9aad2c83c7ef, but the API reference
was not updated accordingly.

This updates the API documentation to match the change.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

api/swagger.yaml

@@ -10414,9 +10414,6 @@ paths:
       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.
@@ -10450,4 +10447,4 @@ paths:
           description: "server error"
           schema:
             $ref: "#/definitions/ErrorResponse"
-      tags: ["Session (experimental)"]
+      tags: ["Session"]

docs/api/version-history.md

@@ -106,6 +106,9 @@ keywords: "API, Docker, rcli, REST, documentation"
 * `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