update docs on how to opt-in (#172)

Author: MarcoGorelli

protocol/purpose_and_scope.md

@@ -50,24 +50,30 @@ aims to develop in the future. When that full API standard is implemented by
 dataframe libraries, the example above can change to:
 
 ```python
-def get_df_module(df):
+def get_compliant_df(df):
     """Utility function to support programming against a dataframe API"""
     if hasattr(df, '__dataframe_namespace__'):
-       # Retrieve the namespace
-       pdx = df.__dataframe_namespace__()
+        # Is already Standard-compliant DataFrame, nothing to do here.
+        pass
+    elif hasattr(df, '__dataframe_standard__'):
+        # Convert to Standard-compliant DataFrame.
+        df = df.__dataframe_standard__()
     else:
         # Here we can raise an exception if we only want to support compliant dataframes,
         # or convert to our default choice of dataframe if we want to accept (e.g.) dicts
-        pdx = pd
-        df = pd.DataFrame(df)
-
-    return pdx, df
+        raise TypeError(
+            "Expected Standard-compliant DataFrame, or DataFrame with Standard-compliant implementation"
+        )
+    return df
 
 
 def somefunc(df, ...):
     """`df` can be any dataframe conforming to the dataframe API standard"""
-    pdx, df = get_df_module(df)
-    # From now on, use `df` methods and `pdx` functions/objects
+    # Get Standard-compliant DataFrame.
+    df = get_compliant_df(df)
+    # Get Standard namespace (optional, only if you need methods from it).
+    namespace = df.__dataframe_namespace__()
+    # From now on, use `df` methods and `namespace` functions/objects
 ```