Fix documentation of the size field in a sigsum proof

The current documentation of sigsum proof includes a field named "tree_size", see https://git.glasklar.is/sigsum/core/sigsum-go/-/blob/357b7b2229ad5d75ecd3d94b36490774329c52a2/doc/sigsum-proof.md#L51

The implementation uses the field name "size", which is consistent wit this section of the proof being a verbatim copy of the get-tree-head response.

The documentation should also say that size=0 is invalid (size=1 is mentioned as a special case, with inclusion proof left out).

Not sure how to deal with the requirement that leaf_index < size, if an out-of-range leaf_index should be rejected as a format error, or checked later when the proof is verified.