describe QUARTO_R, QUARTO_PYTHON, QUARTO_VERSION_REQUIREMENT

Author: cscheid

docs/advanced/environment-vars.qmd

@@ -18,9 +18,9 @@ You can read about other ways to set environment variables in Quarto Projects in
 
 | Variable        | Description |
 |-----------------|-------------|
-| `QUARTO_R`      |             |
-| `QUARTO_PYTHON` |             |
-|                 |             |
+| `QUARTO_R`      | Explicit path to the version of `Rscript` to be used by the `knitr` engine and `quarto run *.R` command. |
+| `QUARTO_PYTHON` | Explicit path to the version of `python` to be used by the `jupyter` engine and `quarto run *.py` command. |
+| `QUARTO_VERSION_REQUIREMENT` | A [`semver`](https://semver.org/) string describing the Quarto version requested by the environment. If this check fails, Quarto will not run. |
 
 ## Variables Quarto sets