Turn Bob::expects into an enum and add docs

This turns the Bob::expects field into an enum, removing the old
constants.  It also makes the field private, nothi0ng outside the
securejoin module uses it.

Finally it documents stuff, including a seemingly-unrelated Param.
But that param is used by the securejoin module... It's nice to have
doc tooltips be helpful.

src/param.rs

@@ -95,6 +95,12 @@ pub enum Param {
     Recipients = b'R',
 
     /// For Groups
+    ///
+    /// An unpromoted group has not had any messages sent to it and thus only exists on the
+    /// creator's device.  Any changes made to an unpromoted group do not need to send
+    /// system messages to the group members to update them of the changes.  Once a message
+    /// has been sent to a group it is promoted and group changes require sending system
+    /// messages to all members.
     Unpromoted = b'U',
 
     /// For Groups and Contacts