Clarify that manhole uses SSH (not Telnet) (#20417)

Spawning from looking at a [security
advisory](https://github.com/element-hq/synapse/security/dependabot/241)
but it doesn't apply because we don't use Telnet. Updating our erroneous
Telnet references to make it more clear that it doesn't apply to us.

Relevant code showing that we initialize a *SSH* listener: https://github.com/element-hq/synapse/blob/bde63a807e81c1b1f36e983b9f1874011181a13d/synapse/util/manhole.py#L74-L117

changelog.d/20417.doc

@@ -0,0 +1 @@
+Clarify config docs that manhole uses SSH (not Telnet).

docs/usage/configuration/config_documentation.md

@@ -613,7 +613,7 @@ listeners:
 ---
 ### `manhole`
 
-*(integer|null)* Turn on the Twisted telnet manhole service on the given port.
+*(integer|null)* Turn on the Twisted manhole service on the given port.
 
 This can also be set by the `--manhole` argument when starting Synapse.
 

schema/synapse-config.schema.yaml

@@ -670,7 +670,7 @@ properties:
   manhole:
     type: ["integer", "null"]
     description: >-
-      Turn on the Twisted telnet manhole service on the given port.
+      Turn on the Twisted manhole service on the given port.
 
 
       This can also be set by the `--manhole` argument when starting Synapse.

synapse/config/server.py

@@ -1088,7 +1088,7 @@ class ServerConfig(Config):
             metavar="PORT",
             dest="manhole",
             type=int,
-            help="Turn on the twisted telnet manhole service on the given port.",
+            help="Turn on the twisted manhole service on the given port.",
         )
 
     def read_gc_intervals(self, durations: Any) -> tuple[float, float, float] | None: