specs: document namespaced padded shares between messages

[evan-forbes]

With the non-interactive defaults, we introduced padding between messages. Per the original specs Caveats section of Message Layout Rational, these are described as

[Namespaced padded shares] have a value of zero and a namespace ID equal to the preceding message's. Since their value is known, they can be omitted from NMT proofs of all shares of a given namespace ID.

We need to make sure that we include these in the share encoding portion of the specs, and update with any implementation modifications.

tracked in #650
blocked by (or should be updated after) #783

[rootulp]

Oops I created a duplicate issue #1247