diff --git a/docs/reference/Image.rst b/docs/reference/Image.rst
index 41d3b8fcec0..66e6b2a0c97 100644
--- a/docs/reference/Image.rst
+++ b/docs/reference/Image.rst
@@ -93,10 +93,14 @@ Generating images
 Registering plugins
 ^^^^^^^^^^^^^^^^^^^
 
+.. autofunction:: preinit
+.. autofunction:: init
+
 .. note::
 
-    These functions are for use by plugin authors. Application authors can
-    ignore them.
+    These functions are for use by plugin authors. They are called when a
+    plugin is loaded as part of :py:meth:`~preinit()` or :py:meth:`~init()`.
+    Application authors can ignore them.
 
 .. autofunction:: register_open
 .. autofunction:: register_mime
diff --git a/src/PIL/Image.py b/src/PIL/Image.py
index 9bf293d13e8..2a6b4646bbd 100644
--- a/src/PIL/Image.py
+++ b/src/PIL/Image.py
@@ -298,7 +298,11 @@ def getmodebands(mode):
 
 
 def preinit():
-    """Explicitly load standard file format drivers."""
+    """
+    Explicitly loads BMP, GIF, JPEG, PPM and PPM file format drivers.
+
+    It is called when opening or saving images.
+    """
 
     global _initialized
     if _initialized >= 1:
@@ -334,11 +338,6 @@ def preinit():
         assert PngImagePlugin
     except ImportError:
         pass
-    # try:
-    #     import TiffImagePlugin
-    #     assert TiffImagePlugin
-    # except ImportError:
-    #     pass
 
     _initialized = 1
 
@@ -347,6 +346,9 @@ def init():
     """
     Explicitly initializes the Python Imaging Library. This function
     loads all available file format drivers.
+
+    It is called when opening or saving images if :py:meth:`~preinit()` is
+    insufficient, and by :py:meth:`~PIL.features.pilinfo`.
     """
 
     global _initialized
@@ -3407,8 +3409,12 @@ def register_open(id, factory, accept=None):
 
 def register_mime(id, mimetype):
     """
-    Registers an image MIME type.  This function should not be used
-    in application code.
+    Registers an image MIME type by populating ``Image.MIME``. This function
+    should not be used in application code.
+
+    ``Image.MIME`` provides a mapping from image format identifiers to mime
+    formats, but :py:meth:`~PIL.ImageFile.ImageFile.get_format_mimetype` can
+    provide a different result for specific images.
 
     :param id: An image format identifier.
     :param mimetype: The image MIME type for this format.