Rephrase part of ?ggplot() docs

[teunbrand]

This PR aims to fix #6445.

To avoid the confusion that it is suggested that fixed aesthetics can also be passed to ggplot(). They cannot.
There was a second suggestion in the issue:

and further down, in "Details":
Static aesthetics can't be set globally with ... like they would in geom_*() functions. To change global static aesthetic values for geometries, use the geom argument in the theme() function.

But I think this is off-topic for the ?ggplot() docs.

[thomasp85]

LGTM