update SEP-0011 to reflect new canonical asset format.

[stanford-scs]

No description provided.

[bartekn]

The part of the doc changed in this PR was referenced in stellar/go#1754 because it's using asset format.

I don't think below is true:

Codes in Asset and AllowTrustOp should escape \, : and any byte outside the range 0x21-0x7e, but no other bytes. Trailing \x00 should not be shown except as needed to show an asset of type ASSET_TYPE_CREDIT_ALPHANUM12 fewer than 5 non-zero bytes.

in the context of stellar-core code. The relevant code is here and it looks like asset code must pass isalnum function so only alphanumeric characters can form a valid asset code.

[MonsieurNicolas]

@bartekn is correct: I don't know where this comes from. Both the code and documentation are basically describing alphanum (with zero bytes only allowed as padding characters at the end of the asset code)