[IMP] developer: document new parameter

closes odoo/documentation#5271

X-original-commit: bf5c888446
Related: odoo/odoo#130863
Signed-off-by: Martin Trigaux (mat) <mat@odoo.com>

content/developer/reference/backend/mixins.rst

@@ -88,6 +88,7 @@ to manage followers on your record:
         parent partners to the message in case of private discussion
     :param list(tuple(str,str)) attachments: list of attachment tuples in the form
         ``(name,content)``, where content is NOT base64 encoded
+    :param bool body_is_html: indicates whether `body` should be treated as HTML, even if `str`.
     :param `\**kwargs`: extra keyword arguments will be used as default column values for the
           new mail.message record
     :return: ID of newly created mail.message