Feature Request: Gallery-wide caption in addition to individual image captions.

It's not currently possible to add a single caption on the entire gallery.

**Request:** Add feature so that users can add a single, gallery-wide, caption.
_Captions can only be added to individual images with in the gallery._

**History:**
This request came through the Jetpack .org forums:
https://wordpress.org/support/topic/feature-request-gallery-captions/

**Example:**
A travel blogger would like to post a "city caption" for their trip to Paris, France - instead of having to label each individual image with a caption.

Start with the issue description and the linked WordPress.org forum thread to understand the requested gallery-wide caption behavior and its intended user experience. Then locate the gallery implementation and existing individual-image caption handling; done requires a decided scope and a working gallery-wide caption without losing current individual captions.

Written by the indexing model from the issue text.