document the new change_channel hook

doc/hook/change_channel.bb

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+[h2]change_channel[/h2]
+
+Called when entering a logged in state in a channel context (as opposed to an account context).
+The hook array provides two arguments, 'channel_id' and 'chanx'. 'chanx' is a union of the channel 
+and xchan records for the now active channel.
+
+Use this to capture what would traditionally be known as 'login events'.  In this platform, login is
+a separate authentication activity and doesn't necessarily require "connecting to an identity", which
+is what the change_channel activity represents. 
+
+