Add information on MSC3873 dotted-path escape rules.

content/appendices.md

@@ -942,6 +942,20 @@ uses the following rules:
 * The character `*` matches zero or more characters.
 * `?` matches exactly one character.
 
+## Dot-separated property paths
+
+It is useful to express the "path" to an event property by concatenating property
+names with dots, e.g. `content.body` would represent the a `body` property in the
+event's `content`.
+
+To handle ambiguity when a property name contains a dot, any literal dot or
+backslash found in a property name should be escaped with a backslash. E.g. a
+property `m.relates_to` in the `content` would be expressed as
+`content.m\.relates_to`.
+
+It is recommended that implementations do not redundantly escape characters, as
+other escape sequences are reserve for future use.
+
 ## Security Threat Model
 
 ### Denial of Service

content/client-server-api/modules/push.md

@@ -214,8 +214,8 @@ The following conditions are defined:
 
 This is a glob pattern match on a property of the event. Parameters:
 
--   `key`: The dot-separated path of the property of the event to match, e.g.
-    `content.body`.
+-   `key`: The [dot-separated path of the property](/appendices#dot-separated property paths)
+    of the event to match, e.g. `content.body`.
 
 -   `pattern`: The [glob-style pattern](/appendices#glob-style-matching) to match against.
 
@@ -297,8 +297,8 @@ For an example of this, see the default rule
 
 This is an exact value match on a property of the event. Parameters:
 
--   `key`: The dot-separated path of the property of the event to match, e.g.
-    `content.body`.
+-   `key`: The [dot-separated path of the property](/appendices#dot-separated property paths)
+    of the event to match, e.g. `content.body`.
 
 -   `value`: The value to match against.
 
@@ -341,8 +341,8 @@ The following `m.federate` values will NOT match:
 
 This matches if an array property of an event exactly contains a value. Parameters:
 
--   `key`: The dot-separated path of the property of the event to match, e.g.
-    `content.body`.
+-   `key`: The [dot-separated path of the property](/appendices#dot-separated property paths)
+    of the event to match, e.g. `content.body`.
 
 -   `value`: The value to match against.