Docs: Clarify multi-select behavior in CollectionAdd and GalleryAdd controllers.

Follow-up to [30120], [31373].

Props yogeshbhutkar, mikachan, luigipulcini, SergeyBiryukov.
Fixes #62696.

git-svn-id: https://develop.svn.wordpress.org/trunk@61447 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/js/media/controllers/collection-add.js

@@ -17,7 +17,9 @@ var Selection = wp.media.model.Selection,
  * @param {object}                     [attributes]                         The attributes hash passed to the state.
  * @param {string}                     [attributes.id=library]              Unique identifier.
  * @param {string}                     attributes.title                     Title for the state. Displays in the frame's title region.
- * @param {boolean}                    [attributes.multiple=add]            Whether multi-select is enabled. @todo 'add' doesn't seem do anything special, and gets used as a boolean.
+ * @param {boolean|string}             [attributes.multiple=add]            Whether multi-select is enabled. Accepts 'add' or true.
+ *                                                                          When set to true, requires Shift or Cmd/Ctrl to select multiple items.
+ *                                                                          When set to 'add', allows selecting multiple items by clicking thumbnails.
  * @param {wp.media.model.Attachments} [attributes.library]                 The attachments collection to browse.
  *                                                                          If one is not supplied, a collection of attachments of the specified type will be created.
  * @param {boolean|string}             [attributes.filterable=uploaded]     Whether the library is filterable, and if so what filters should be shown.

src/js/media/controllers/gallery-add.js

@@ -20,7 +20,9 @@ var Selection = wp.media.model.Selection,
  * @param {Object}                     [attributes]                         The attributes hash passed to the state.
  * @param {string}                     [attributes.id=gallery-library]      Unique identifier.
  * @param {string}                     [attributes.title=Add to Gallery]    Title for the state. Displays in the frame's title region.
- * @param {boolean}                    [attributes.multiple=add]            Whether multi-select is enabled. @todo 'add' doesn't seem do anything special, and gets used as a boolean.
+ * @param {boolean|string}             [attributes.multiple=add]            Whether multi-select is enabled. Accepts 'add' or true.
+ *                                                                          When set to true, requires Shift or Cmd/Ctrl to select multiple items.
+ *                                                                          When set to 'add', allows selecting multiple items by clicking thumbnails.
  * @param {wp.media.model.Attachments} [attributes.library]                 The attachments collection to browse.
  *                                                                          If one is not supplied, a collection of all images will be created.
  * @param {boolean|string}             [attributes.filterable=uploaded]     Whether the library is filterable, and if so what filters should be shown.