Update Gaia API documents for #c1ed307f23ecdb82885734f20c1b36fce05b25f5
    Bug 951339 - [Messages][Drafts] Existing thread drafts not properly populated when a new message is sent to that same recipient r=julienw

https://bugzilla.mozilla.org/show_bug.cgi?id=951339
Signed-off-by: Rick Waldron <waldron.rick@gmail.com>