Document scrolling for hover command in keymap.md (#1117)

* Document scrolling for hover command in keymap.md

* Move popup keys to a dedicated section

book/src/keymap.md

@@ -214,7 +214,7 @@ This layer is a kludge of mappings, mostly pickers.
 | -----   | -----------                                                             | -------                             |
 | `f`     | Open file picker                                                        | `file_picker`                       |
 | `b`     | Open buffer picker                                                      | `buffer_picker`                     |
-| `k`     | Show documentation for item under cursor (**LSP**)                    | `hover`                             |
+| `k`     | Show documentation for item under cursor in a [popup](#popup) (**LSP**) | `hover`                             |
 | `s`     | Open document symbol picker (**LSP**)                                   | `symbol_picker`                     |
 | `S`     | Open workspace symbol picker (**LSP**)                                  | `workspace_symbol_picker`           |
 | `r`     | Rename symbol (**LSP**)                                                 | `rename_symbol`                     |
@@ -230,6 +230,15 @@ This layer is a kludge of mappings, mostly pickers.
 
 > TIP: Global search displays results in a fuzzy picker, use `space + '` to bring it back up after opening a file.
 
+##### Popup
+
+Displays documentation for item under cursor.
+
+| Key      | Description |
+| ----     | ----------- |
+| `Ctrl-u` | Scroll up   |
+| `Ctrl-d` | Scroll down |
+ 
 #### Unimpaired
 
 Mappings in the style of [vim-unimpaired](https://github.com/tpope/vim-unimpaired).