Warning, this isn't the "detailed documentation" you wanted, just a simple answer.
It's actually pretty simple (quotes are from: https://help.ubuntu.com/community/UbuntuUpdates)
- The security updates are holes in software that need to be patched for, obviously,
Important security updates
These are obviously the only ones you will have to enable if your
computer is connected to the Internet, in order to keep you secure.
Enabling this is smart. Even if you don't connect to the Internet, it
Updates (Recommended Updates):
- The Recommended Updates are just software changes, but not updates that will affect the security of your system, just the apps installed and stuff like that. They tend to fix bugs and annoying problems as well as add features.
These updates contain non-critical updates which can remove major
annoyances and broken packages, but which do not affect your security.
Other than fixing some, they do not enable any features.
Enabling this is generally a good idea. The amount to download as well
as the changes are not too big, but it improves your system stability
in various ways.
Proposed: (For getting married with :D Just kidding)
The Proposed updates are updates that have to be tested before they can be officially put into the Recommended updates repository. They may have issues that could break your system, and quite often do. You get Beta features, but often a broken system. I managed to nuke an installation by enabling this. Unless you like testing and fixing issues, I don't recommended this one.
There is a question regarding this here: What is the "proposed" repository?
Enabling the proposed updates repository can break your
system. It is not recommended for inexperienced users.
The proposed updates are updates which are waiting to be moved into
the recommended updates queue after some testing. They may never reach
recommended or they may be replaced with a more recent update.
Enabling this is reasonable if you want to participate in testing
minor updates, or know that your specific problem has been solved here
but the package hasn't reached recommended yet.
Backported updates are pieces of software which come from a newer
major release. Thus, they can contain new features, but may also break
compatibility with their older version. However, they are compiled
specifically for your version of Ubuntu. In effect it saves you the
hassle of broken dependencies and major downloads.
Enabling this is reasonable if you want new features but don't want
your system to be unstable.
Read more in UbuntuBackports.