Clarify that `base_url` in client `well_known` may or may not include trailing slash (#3562)

Fixes #3465. The change adds a line explicitly warning developers that the base_url of client well_known may or may not include a trailing slash and to be prepared to handle both cases.

changelogs/client_server/newsfragments/3562.clarification

@@ -0,0 +1 @@
+Clarify that `base_url` in client `well_known` may or may not include trailing slash.

content/client-server-api/_index.md

@@ -297,6 +297,8 @@ specify parameter values. The flow for this method is as follows:
             done as a simple check against configuration errors, in
             order to ensure that the discovered address points to a
             valid homeserver.
+        3. It is important to note that the `base_url` value might include
+           a trailing `/`. Consumers should be prepared to handle both cases.
     6.  If the `m.identity_server` property is present, extract the
         `base_url` value for use as the base URL of the identity server.
         Validation for this URL is done as in the step above, but using