Specify some of the common concepts for Matrix in the index

See https://github.com/matrix-org/matrix-doc/pull/2061
Fixes https://github.com/matrix-org/matrix-doc/issues/1468
Fixes https://github.com/matrix-org/matrix-doc/issues/1528

The section is not referenced by the specifications yet - they do a fairly good job of explaining it over and over. In future, it would be good to point all the references to the index.

api/server-server/keys_query.yaml

@@ -46,6 +46,8 @@ paths:
             **Deprecated**. Servers should not use this parameter and instead
             opt to return all keys, not just the requested one. The key ID to
             look up.
+
+            When excluded, the trailing slash on this endpoint is optional.
           required: false
           x-example: "ed25519:abc123"
         - in: query

api/server-server/keys_server.yaml

@@ -51,6 +51,8 @@ paths:
             **Deprecated**. Servers should not use this parameter and instead
             opt to return all keys, not just the requested one. The key ID to
             look up.
+
+            When excluded, the trailing slash on this endpoint is optional.
           required: false
           x-example: "ed25519:abc123"
           deprecated: true

changelogs/server_server/newsfragments/2097.clarification

@@ -0,0 +1 @@
+Clarify that the trailing slash is optional on ``/keys/*`` endpoints when no key ID is requested.

specification/index.rst

@@ -425,6 +425,42 @@ dedicated API.  The API is symmetrical to managing Profile data.
   Would it really be overengineered to use the same API for both profile &
   private user data, but with different ACLs?
 
+
+Common concepts
+---------------
+
+Various things are common throughout all of the Matrix APIs. They are
+documented here.
+
+Trailing slashes on API endpoints
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Unless the endpoint is explicltly specified to have a trailing slash, the
+slash is optional. For example, an endpoint specified as ``/_matrix/example/``
+would require a trailing slash, however an endpoint specified as ``/_matrix/example``
+has an optional slash (can be excluded when making requests).
+
+Namespacing
+~~~~~~~~~~~
+
+Namespacing helps prevent conflicts between multiple applications and the specification
+itself. Where namespacing is used, ``m.`` prefixes are used by the specification to
+indicate that the field is controlled by the specification. Custom or non-specified
+namespaces used in the wild SHOULD use the Java package naming convention to prevent
+conflicts.
+
+As an example, event types are namespaced under ``m.`` in the specification however
+any client can send a custom event type, such as ``com.example.game.score`` without
+needing to put the event into the ``m.`` namespace.
+
+Timestamps
+~~~~~~~~~~
+
+Unless otherwise stated, timestamps are measured as milliseconds since the Unix epoch.
+Throughout the specification this may be referred to as POSIX, Unix, or just "time in
+milliseconds".
+
+
 .. _`room versions`:
 
 Room Versions