Update spec doc in light of discussion

Author: markt-asf

spec/src/main/asciidoc/WebSocket.adoc

@@ -600,10 +600,15 @@ order in which extensions are presented in the opening handshake must be
 maintained as the order determines the order in which the extensions will
 process WebSocket messages.
 
-Any extension whose name is not included in the list of extensions returned by
+Any extension whose name is returned from `ServerEndpointConfig.getExtensions()`
+where the name is prefixed with a `<`, must be excluded from the opening
+handshake response.
+
+If `ServerContainer.getInstalledExtensions()` (excluding extensions with names
+prefaced with `<`) is a non-empty list, any extension whose name is not
+included in the list of extensions returned by
 `ServerContainer.getInstalledExtensions()` must be excluded from the opening
-handshake response. Implementation and configuration of supported extensions is
-container specific.
+handshake response.
 
 The client may include the same extension multiple times. This signifies that
 different configurations of the extension are acceptable with the variations
@@ -873,6 +878,16 @@ the endpoint [WSC-4.1.5-1]. The developer may use this technique to
 share state across all instances of the endpoint in addition to
 customizing the opening handshake.
 
+==== extensionConfigs
+
+The optional extensionConfigs attribute allows the developer to indicate which
+WebSocket extensions this endpoint is prepared to accept if the extension is
+requested by the client and it is supported by the server container. If an
+extension name is prefaced with `<` that indicates that the extension must not
+be used regardless of whether the client requests it and/or the server container
+supports it. Listing the same extension with and without the `<` prefix is not
+valid and will trigger a `DeploymentException` on deployment.
+
 [[clientendpoint]]
 === @ClientEndpoint
 
@@ -933,6 +948,17 @@ implementation must use this list in the opening handshake to negotiate
 the desired subprotocol to use for the connection it establishes
 [WSC-4.2.4-1].
 
+==== extensionConfigs
+
+The optional extensionConfigs attribute allows the developer to indicate which
+WebSocket extensions this endpoint would like to request if the extension is
+supported by the client and server containers and permitted by the server
+endpoint. Extensions should be listed in the order they should be used. The same
+extension may appear multiple times with different configurations. Where an
+extension is listed with multiple configurations, the configurations must be
+provided in preference order. Extensions should be listed in preference order
+and the same extension may appear multiple times with different configurations.
+
 [[pathparam]]
 === @PathParam