[6.x] Harmonize label customization config text

[duncanmcclean]

This pull request harmonizes the config text around setting/customizing labels across fieldtypes, as suggested in #14741.

• Replicator's "Add Set Label" instructions now follow the Grid/List pattern: Customize the label of the "Add Set" button.
• Revealer's "Input Label" instructions now follow along and reference its actual default label: Customize the label of the "Show Fields" button, or the label beside the toggle.
• The List fieldtype's config option is now displayed as "Add Item Label", matching its button which says "Add Item" (it was previously "Add Row Label").
• The Array fieldtype's "Keys" instructions now match Checkboxes/Radio: Set the array keys (variables) and their optional labels.
• This PR also gives the Grid and Replicator label configs placeholders showing their default button labels ("Add Row" / "Add Set"), matching the List fieldtype's existing "Add Item" placeholder.

Closes #14741

[ebeauchamps]

Lovely