Update Gaia API documents for #8f405c725df8dd83afa0b0b9307ba0c80364aa40
    Bug 952225 - [Messages][Drafts] Support subject line in message drafts r=julienw

https://bugzilla.mozilla.org/show_bug.cgi?id=952225
Signed-off-by: Rick Waldron <waldron.rick@gmail.com>