Overlays large, semi-transparent, rotated text across a ggplot or
xpose_plot object (e.g. "DRAFT", "PRELIMINARY", "CONFIDENTIAL").
Calling it directly always adds a watermark (falling back to the
built-in defaults below if nothing else is configured). It is also
applied automatically by print.xpose_plot() and
ggsave_xp() whenever a default_watermark is actually configured
(see the auto_apply entry in set_xtras_options()) -- unlike this
function, that automatic trigger never invents an unconfigured
watermark on its own.
label/colour/alpha/size/angle/fontface resolve with the same
precedence as apply_default_labs(): (in increasing precedence) the
xpose.xtras.default_watermark R option, xpdb-level defaults set via
set_default_watermark() (when xpdb is supplied), then the argument
itself when explicitly passed. Built-in fallbacks ("DRAFT",
"grey50", 0.3, 24, 30, "bold") apply if a setting isn't
resolved from any of those.