[UX] Layout UI: Improve the help/description text for the "Main content" block in the case of layouts that allow field blocks to be placed

[klonos]

This came up in various discussions triggered by recent activity/momentum in #2894. This UX task here is related to that issue, but the problem to address is pre-existing, and well-described by @stpaultim in the sibling issue #6460 (which tries to address it with new configuration options provided during specific workflows), as well as a feedback comment in #2894 (comment):

I think we should add some text somewhere that says something to this effect - "Adding field blocks may result in duplication of content unless matching field is hidden on the default display for the entity type."

This issue here is to improve the UX/UI in the "Manage blocks" UI, by adding new messages and/or help text, or improving existing ones.

No solid proposal has been formulated yet, but I have certain ideas, so creating this issue here as a placeholder for now. Here's what I have currently:

There could be a message once a layout for a specific fieldable entity/bundle has been created, or a generic warning shown in layouts where both the "Main content" block as well as individual field blocks have been placed. It could even be added in the text of the "Main content" block. Something like this:

By default, this block allows displaying all fields [for all content types|for the content type XYZ] collectively (fields can be managed in the respective "Manage display" [link] form).

Then, depending on whether certain field blocks have already been individually placed in the layout or not yet, we could be showing either of these following texts in addition to the help text above:

• If field blocks have not been placed yet in the layout:

  It is however possible to select individual field blocks to be managed independently of the Main content block (for example, a common use case is to place certain blocks in a sidebar region on the layout).

• If field blocks have already been placed in the layout:

  Certain field blocks have been placed individually in this layout, independently of the Main content block. This is known to cause duplication of content. You may need to adjust ...

Still trying to fully flesh this out ...more updates to come (and even a PR sandbox to test these proposals).