The tooltip for the format contains the following text: "The format describes the date and time parts to insert into the file name. " "For example, %Y will be substituted with the year, %m with the month and %d " "with the day. See the documentation of the date utility for additional " "information." "to insert into the file name" is wrong, it should be corrected with "to insert into the panel". And as we are at it, we may just as well describe it's possible to use tags like <span font_desc="16"></span> and <b></b>.
Fixed the typo in be15b49. Special markup should be mentioned in the manual.
Fixed in docs.xfce.org.