describe requires-python fallback in docs

crates/ruff_workspace/src/options.rs

@@ -312,7 +312,9 @@ pub struct Options {
     /// ```
     ///
     /// If both are specified, `target-version` takes precedence over
-    /// `requires-python`.
+    /// `requires-python`. See [_Inferring the Python version_](https://docs.astral.sh/ruff/configuration/#inferring-the-python-version)
+    /// for a complete description of how the `target-version` is determined
+    /// when left unspecified.
     ///
     /// Note that a stub file can [sometimes make use of a typing feature](https://typing.readthedocs.io/en/latest/spec/distributing.html#syntax)
     /// before it is available at runtime, as long as the stub does not make

docs/configuration.md

@@ -307,6 +307,17 @@ If Ruff detects multiple configuration files in the same directory, the `.ruff.t
 precedence over the `ruff.toml` file, and the `ruff.toml` file will take precedence over
 the `pyproject.toml` file.
 
+### Inferring the Python version
+When no discovered configuration specifies a [`target-version`](settings.md#target-version), Ruff will attempt to fall back to the minimum version compatible with the `requires-python` field in a nearby `pyproject.toml`.
+The rules for this behavior are as follows:
+
+1. If a configuration file is passed directly, Ruff does not attempt to infer a missing `target-version`.
+1. If a configuration file is found in the filesystem hierarchy, Ruff will infer a missing `target-version` from the `requires-python` field in a `pyproject.toml` file in the same directory as the found configuration.
+1. If we are using a user-level configuration from `${config_dir}/ruff/pyproject.toml`, the `requires-python` field in the first `pyproject.toml` file found in an ancestor of the current working directory takes precedence over the `target-version` in the user-level configuration.
+1. If no configuration files are found, Ruff will infer the `target-version` from the `requires-python` field in the first `pyproject.toml` file found in an ancestor of the current working directory.
+
+Note that in these last two cases, the behavior of Ruff may differ depending on the working directory from which it is invoked.
+
 ## Python file discovery
 
 When passed a path on the command-line, Ruff will automatically discover all Python files in that