add docs

Author: aignas

docs/sphinx/pip.md

@@ -85,16 +85,19 @@ For example:
     },
     # For the list of standard platforms that the rules_python has toolchains for, default to
     # the following requirements file.
-    "requirements_lock" = "requirements_lock.txt",
+    requirements_lock = "requirements_lock.txt",
 ```
 
+In case of duplicate platforms, `rules_python` will raise an error as there has
+to be unambiguous mapping of the requirement files to the (os, arch) tuples.
+
 An alternative way is to use per-OS requirement attributes.
 ```starlark
     # ...
+    requirements_windows = "requirements_windows.txt",
     requirements_darwin = "requirements_darwin.txt",
-    # For the list of standard platforms that the rules_python has toolchains for, default to
-    # the following requirements file.
-    "requirements_lock" = "requirements_lock.txt",
+    # For the remaining platforms (which is basically only linux OS), use this file.
+    requirements_lock = "requirements_lock.txt",
 )
 ```