docs: document some environment variables (#2077)

This is so they're more discoverable for both developers and users.

docs/sphinx/environment-variables.md

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+# Environment Variables
+
+:::{envvar} RULES_PYTHON_REPO_DEBUG
+
+When `1`, repository rules will print debug information about what they're
+doing. This is mostly useful for development to debug errors.
+:::
+
+:::{envvar} RULES_PYTHON_REPO_DEBUG_VERBOSITY
+
+Determines the verbosity of logging output for repo rules. Valid values:
+
+* `DEBUG`
+* `INFO`
+* `TRACE`
+:::
+
+:::{envvar} RULES_PYTHON_PIP_ISOLATED
+
+Determines if `--isolated` is used with pip.
+
+Valid values:
+* `0` and `false` mean to not use isolated mode
+* Other non-empty values mean to use isolated mode.
+:::
+
+:::{envvar} RULES_PYTHON_BZLMOD_DEBUG
+
+When `1`, bzlmod extensions will print debug information about what they're
+doing. This is mostly useful for development to debug errors.
+:::
+
+:::{envvar} RULES_PYTHON_ENABLE_PYSTAR
+
+When `1`, the rules_python Starlark implementation of the core rules is used
+instead of the Bazel-builtin rules. Note this requires Bazel 7+.
+:::
+
+:::{envvar} RULES_PYTHON_BOOTSTRAP_VERBOSE
+
+When `1`, debug information about bootstrapping of a program is printed to
+stderr.
+:::
+
+:::{envvar} VERBOSE_COVERAGE
+
+When `1`, debug information about coverage behavior is printed to stderr.
+:::

docs/sphinx/index.md

diff --git a/docs/sphinx/index.md b/docs/sphinx/index.md
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@@ -66,6 +66,7 @@
 support
 Changelog <changelog>
 api/index
+environment-variables
 glossary
 genindex
 ```