Fix description of entry points mechanism

[ashwinvis]

• Show how a module would be loaded. A dictionary of modules will not be loaded, unless created by the user. The example code was corrected.
• Update links to external documentation
• Hint that specifying entry points is not limited to setuptools. For example flit and poetry support it too.

Following up on #870 and #879

[ashwinvis]

Oh no.... you merged it too soon @webknjaz 😅 !

I wanted to add this change:

From d16106c05e1e10f2821da5bdead7dbd5c691aba6 Mon Sep 17 00:00:00 2001
From: Ashwin Vishnu <y4d71nsar@relay.firefox.com>
Date: Thu, 6 May 2021 18:56:29 +0200
Subject: [PATCH] Clarify EntryPoints vs EntryPoint

---
 source/guides/creating-and-discovering-plugins.rst | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/source/guides/creating-and-discovering-plugins.rst b/source/guides/creating-and-discovering-plugins.rst
index 8fe1e41..ebdf876 100644
--- a/source/guides/creating-and-discovering-plugins.rst
+++ b/source/guides/creating-and-discovering-plugins.rst
@@ -151,7 +151,9 @@ Then you can discover and load all of the registered entry points by using
     discovered_plugins = entry_points(group='myapp.plugins')
 
 
-In this example, ``discovered_plugins`` would be a collection of type :class:`importlib.metadata.EntryPoints`:
+In this example, ``discovered_plugins`` would be an
+:class:`importlib.metadata.EntryPoints` object, which is a sequence of all
+``EntryPoint`` objects belonging to the selected group.
 
 .. code-block:: python
 
-- 
2.31.1

[webknjaz]

@ashwinvis oh, you can send that in a follow-up PR. Also, if you want to mark a PR as not ready for review, just make it a Draft PR (normally you should be able to switch between draft and normal PR state at any point), or at least mark it as a WIP in the title — this will be a signal for the maintainers to postpone reviewing/merging it.