Merge pull request #2000 from larrybradley/import-warn

Put module import message in a warning box

docs/getting_started/importing.rst

@@ -24,12 +24,17 @@ or::
     >>> from photutils.aperture import CircularAperture
     >>> aper = CircularAperture((10, 20), r=4)
 
-In general, one should not import from specific submodules of packages.
-This is unnecessary and the internal organization of the package may
-change without notice. For example, the following is not recommended::
 
-    >>> from photutils.aperture.circle import CircularAperture
-    >>> aper = CircularAperture((10, 20), r=4)
+.. warning::
+
+    *Do not import from specific modules of packages.* This is
+    unnecessary and the internal organization of the package may change
+    without notice. All public tools are available in the package
+    top-level namespace. For example, do not import from the ``circle``
+    module within the ``aperture`` package::
+
+        >>> from photutils.aperture.circle import CircularAperture
+        >>> aper = CircularAperture((10, 20), r=4)
 
 
 .. warning::