Add docs warning

Signed-off-by: Filipe Laíns <lains@riseup.net>

Author: FFY00

Doc/library/importlib.rst

@@ -283,6 +283,16 @@ ABC hierarchy::
 
       Returns an iterable of possible specs.
 
+      .. warning::
+         This method can potentially yield a very large number of objects, and
+         it may carry out IO operations when computing these values.
+
+         Because of this, it will generaly be desirable to compute the result
+         values on-the-fly, as they are needed. As such, the returned object is
+         only guaranteed to be an :class:`iterable <collections.abc.Iterable>`,
+         instead of a :class:`list` or other
+         :class:`collection <collections.abc.Collection>` type.
+
       .. versionadded:: next
 
 
@@ -325,6 +335,16 @@ ABC hierarchy::
 
       Returns an iterable of possible specs.
 
+      .. warning::
+         This method can potentially yield a very large number of objects, and
+         it may carry out IO operations when computing these values.
+
+         Because of this, it will generaly be desirable to compute the result
+         values on-the-fly, as they are needed. As such, the returned object is
+         only guaranteed to be an :class:`iterable <collections.abc.Iterable>`,
+         instead of a :class:`list` or other
+         :class:`collection <collections.abc.Collection>` type.
+
       .. versionadded:: next