Fill in @defgroup/topic docs for Extending Drupal

This is looking great. Thanks for taking the time to write this up and to update the existing hooks section. I have just a few comments, all of which are just my gut reaction to the documentation so feel free to ignore them if you don't agree. With the exception of one missing period, and what I think is a critical link that should be added. :)

1. +++ b/core/includes/module.inc
   @@ -178,31 +178,46 @@ function module_uninstall($module_list = array(), $uninstall_dependents = TRUE)
   + * - Change the name of the function, substituting your module's short name
   + *   for the "hook" part of the sample function name. For instance, to implemnt
   + *   hook_batch_alter(), you would rename it to my_module_batch_alter().
   

   I think we need to make sure and explain "module's short name" and how I know what that is for my module. Might even be worthwhile to provide an example mapping "example_module.info" to example_module_batch_alter().

2. +++ b/core/includes/module.inc
   @@ -178,31 +178,46 @@ function module_uninstall($module_list = array(), $uninstall_dependents = TRUE)
   + * To invoke a hook, use methods on
   

   Think it's worth adding something that explains why a module might want to "invoke" a hook? Making it possible for other module's to extend your own module, etc.

3. +++ b/core/modules/system/core.api.php
   @@ -452,15 +452,63 @@
   + * @defgroup extending Extending and altering Drupal
   

   Missing period at end of line.

4. +++ b/core/modules/system/core.api.php
   @@ -452,15 +452,63 @@
   + * Overview of add-ons and alteration methods for Drupal
   

   Missing period at end of line.

5. +++ b/core/modules/system/core.api.php
   @@ -452,15 +452,63 @@
   + * @section extend_types Alteration mechanisms modules can use
   

   I think something like "Extending and Altering Drupal with Modules" or even just "Module basics" might be a better title for this section. Right now it doesn't tell me that this is also about "extending".

6. +++ b/core/modules/system/core.api.php
   @@ -452,15 +452,63 @@
   + * Here is a list of the mechanisms and methods that modules can use to alter
   + * or extend Drupal's core behavior, or the behavior of other modules:
   

   This sentence can probably be simplified to just "Here is a list of ways that modules can alter or extend ...". It seems a little wordy as is. But that's just my gut reaction.

7. +++ b/core/modules/system/core.api.php
   @@ -452,15 +452,63 @@
   + *   @link entity_api Entity API topic @endlink for more information. Note that
   + *   there are also entity hooks and plugin types that you can use to add
   + *   entity bundles or sub-types, alter display, etc.
   

   The note about sub-types and hooks doesn't seem necessary here. I feel like we can just explain that in the entity api docs.

+++ b/core/modules/system/core.api.php
@@ -452,15 +452,63 @@
+ * - Modules: Modules add to or alter the behavior and functionality of Drupal,
+ *   by using one or more of the methods listed in the section below.

I think we should add a link to https://drupal.org/developing/modules/8 in this section.

This looks good to me. Thanks for fixing up those couple of nit-picks.

OMFG that is so much better I can't even explain it. I especially like having a nice list of "extension points" for Drupal 8, since this is a lot different than in 7 and below. Great work!

Committed and pushed to 8.x. Thanks!