Document various enums and flags

[Mr0grog]

In writing code for setting account flags, it would be nice to be able to use the relevant enum:

import static org.stellar.sdk.xdr.AccountFlags.*;
new SetOptionsOperation.Builder()
    .setSetFlags(AUTH_REQUIRED_FLAG.getValue() | AUTH_REVOCABLE_FLAG.getValue())
    .build();

// the above is more clear than:
new SetOptionsOperation.Builder()
    .setSetFlags(1 | 2)
    .build();

…but the XDR bits are not documented, so someone wouldn't know those exist without browsing the source. It would be nice if these were documented somehow (not sure how easy that is since the XDR is generated).