[Rd] documentation for "Startup" should be clearer about system Rprofile (PR#8969)

[htang at hpl.hp.com]

Full_Name: Hsiu-Khuern Tang
Version: 2.3.0
OS: Debian GNU/Linux
Submission from: (NULL) (15.243.169.73)


On reading ?Startup, I cannot find anything that says that
R_HOME/library/base/R/Rprofile is always run.  The "Note" section does say that
R_HOME/etc/Renviron is distinct from the site-wide Renviron file, so I think
this is a natural place to also mention that base/R/Rprofile is distinct from
the site-wide Rprofile file.  In addition, it would be helpful to indicate when
base/R/Rprofile is run: after reading the Renviron files and before reading the
site-wide Rprofile file.

I suppose one possible reason for not including this information is that users
are not supposed to alter base/R/Rprofile.

Thanks,
Hsiu-Khuern.