Make sure description of devtools::check() is accurate re: NOT_CRAN

[jennybc]

devtools::check() does NOT in fact set NOT_CRAN. Some of the most common ways of running devtools::check() do set NOT_CRAN=true: namely, the RStudio Build pane and r-travis. But NOT_CRAN will not be set for a call to devtools::check() in the Console, unless the user has made explicit effort to do so.

r-lib/devtools#2135

Rewording is needed in this vicinity:

r-pkgs/check.Rmd

Line 25 in 1c6b34e

* Sets the `NOT_CRAN` environment variable to `TRUE`. This allows you to

[jimhester]

Actually after r-lib/devtools@4663004 devtools::check() will set NOT_CRAN now. If we want I can do a devtools release before rstudio::conf with this change.

[jennybc]

OK great @jimhester. I'll leave the release timing to you, but it's clear that the wording in R Packages is faithful to devtools past and current intent and, in due course, the next devtools release.