Update `AccountRoot` `lsfGlobalFreeze` flag description

[Tapanito]

The AccountRoot documentation for describes the lsfGlobalFreeze flag as "All assets issued by this account are frozen.".

With the introduction of MPTs this is no longer the case. lsfGlobalFreeze only affects IOUs, not MPTs. Therefore, the documentation should reflect that lsfGlobalFreeze will only effect IOUs issued by the account.

cc @sappenin