document the target_platforms attribute

Author: aignas

docs/pypi/download.md

@@ -113,6 +113,21 @@ This will not work for sdists with C extensions, but pure Python sdists may stil
 approach.
 :::
 
+By default we select the host `{os}_{arch}` platform defined in the `rules_python` `MODULE.bazel`
+file. This means that `rules_python` by default does not come with cross-platform building support
+because some packages have very large wheels and users should be able to use `bazel query` with
+minimal overhead. As a result, we think that it is up to the user to configure the `pip.parse`
+calls and select which platforms they want to target via
+{obj}`pip.parse.target_platforms` attribute:
+```starlark
+    # Enable free threaded and non-freethreaded switching on the host platform
+    target_platforms = ["{os}_{arch}", "{os}_{arch}_freethreaded"],
+
+    # Enable building for `linux_x86_64` containers and the host platform.
+    target_platforms = ["{os}_{arch}", "linux_x86_64"],
+)
+```
+
 ### Using `download_only` attribute
 
 Let's say you have two requirements files: