Fix "Configuring the WebSocket Engine" documentation

This commit updates reference documentation in order
to specify that ServletServerContainerFactoryBean is for
server side use and WebSocketContainerFactoryBean
for client side use.

Issue: SPR-11565
Document locale convention in Tiles def file names

In Tiles v3 integration, underscores in filenames are
intended to identify the definition locale. This behavior
is now documented in order to avoid unexpected results
with filenames like tiles_definitions.xml.

This commit also updates Tiles v2 references to Tiles v3
in the Spring reference documentation.

Issue: SPR-11491