[Global Styles] Provide guidance on how to add per-block CSS

[mrfoxtalbot]

What problem does this address?

When adding custom CSS styles on a per-block basis, only the property:value needs to be added, no selector. This is has been updated in the documentation but:

• It is very counterintuitive and users are unlikely to figure it right away.
• The support doc the "Learn more about CSS" links to is outdated and does not mention per-block CSS (@kathrynwp has added a hot fix, see Outdated content and links to Codex in CSS Documentation-Issue-Tracker#424 (comment)).

What is your proposed solution?

1. I would include some context in the description. Something to the effect of "Do not add a CSS selector, just add the property:value.
2. Add a placeholder in the textarea to provide an example.

[jordesign]

I've just opened a PR to add some additional instructions in the label. It's only part of the solution but might help some.

Not sure who would be best to get to review this though? I'm guessing the actual wording could also be refined.

Pinging @kathrynwp for an FYI and any suggestions.

[jordesign]

Oh - and another PR which brings in some placeholder text.
#49627

@glendaviesnz @tellthemachines - I think both of you have worked on the Custom CSS functionality - I'd be interested in your thoughts (and the proposed additions) to make the correct syntax clearer (for per-block CSS)

[richtabor]

Perhaps we could leverage the Tip UI like seen in the Inserter.

[mtias]

placeholder example sounds good

[jordesign]

I'd be keen to see that happen. I've tried working on a PR for it myself - but can't figure out how to add a placeholder to the ` element...