Remove "experimental" from TLS docs [skip ci]
TLS/SSL
=======
-Currently dask distributed has experimental support for TLS/SSL communication,
+Dask distributed has support for TLS/SSL communication,
providing mutual authentication and encryption of communications between cluster
-endpoints (Clients, Schedulers and Workers).
+endpoints (Clients, Schedulers, and Workers).
TLS is enabled by using a ``tls`` address such as ``tls://`` (the default
being ``tcp``, which sends data unauthenticated and unencrypted). In
