Not a data constructor clarity

For a module containing only:
```hs
data Foo = bar
```
we get the error message:
```
error: Not a data constructor: ‘bar’
|
1 | data Foo = bar
|
```
when compiling or loading into ghci. It came up that this message does not explain why 'bar' is not a valid data constructor, that it does not start with a capital letter or ':'. It would be useful to have that information presented alongside this error message, perhaps with something like:
```
error: Not a data constructor: ‘bar’
Note: data constructors must begin with a capital letter or ':'
|
1 | data Foo = bar
|
```
or an equivalent that better matches the style of GHC's other error messages. If adding an additional note would not fit that style, simply changing the message to:
```
error: Not a valid data constructor: ‘bar’
|
1 | data Foo = bar
|
```
would help make it more clear that 'bar' _cannot_ be a data constructor and that this is a syntactical issue. I believe there are no other methods of raising this error, but I could see either approach making such cases more confusing if they do exist.

Reproduce the diagnostic by compiling or loading the shown `data Foo = bar` snippet into GHCi, then trace where the “Not a data constructor” message is produced. Done means the diagnostic clearly explains the capitalization or `:` requirement, with the behavior covered by the relevant compiler error test if one is found.

Written by the indexing model from the issue text.