Block hooks: add short description to Plugins section

[annezazu]

Currently, there's no context when a block is added via a plugin thanks to block hooks:

To help add some clarity, can we add in some short text describing what's going on? Here are some ideas:

• These blocks are suggested automatically by your active plugins.
• These blocks are automatically included by your active plugins.
• These blocks are automatically added by your active plugins.

Here's how this might look:

We might want to consider making it a "hint" similar to other spots in the interface as it may not be something people need to see regularly described:

@WordPress/gutenberg-design for thoughts here!