docs: spec tail padding share

[rootulp]

Closes #1175

See the implementation here.

Screenshot

[cmwaters]

LGTM.

Does it make sense to add here how tail padding is used rather than just what it is?

[rootulp]

I think it would help. How about?

Tail padding shares are placed after the last blob in the data square so that the number of shares in the data square is a perfect square. Clients can safely ignore the contents of these shares because they don't contain any significant data.

Although this just made me wonder: what if a block proposer includes tail padding shares with the tail padding namespace but actual data populated in the rawData? Do we want to check for and explicitly reject blocks with such shares?

[cmwaters]

Thank you!