- remove cutoff for picking RSPM, always use it
- remove now-redundant r4.0-mran test
- shift base R 3.6 snapshot date to after October 2017, when rspm snapshots seem to have started
- update docs to point to RSPM
- clarify that runtime.txt is only for env files that don't support runtime specification
- specify default version of Python and when it changed
- more links to pinning docs
Stops and starts emphasis around an inline code markup.
Also removes two emphasis sections, since these are at the start of
bullet points, creating emphasis already.