add docs for new term:showsplitbuttons config key (#3160)

Author: sawka

docs/docs/config.mdx

@@ -76,6 +76,7 @@ wsh editconfig
 | term:bellindicator <VersionBadge version="v0.14" />                  | bool     | when enabled, shows a visual indicator in the tab when the terminal bell is received (default false)                                                                                                                                                           |
 | term:osc52 <VersionBadge version="v0.14" />                          | string   | controls OSC 52 clipboard behavior: `always` (default, allows OSC 52 at any time) or `focus` (requires focused window and focused block)                                                                                                                     |
 | term:durable <VersionBadge version="v0.14" />                        | bool     | makes remote terminal sessions durable across network disconnects (defaults to false)                                                                                                                                                                           |
+| term:showsplitbuttons <VersionBadge version="v0.15" />               | bool     | when enabled, shows split horizontal and vertical buttons in the terminal block header (defaults to false)                                                                                                                                                      |
 | editor:minimapenabled                | bool     | set to false to disable editor minimap                                                                                                                                                                                                                        |
 | editor:stickyscrollenabled           | bool     | enables monaco editor's stickyScroll feature (pinning headers of current context, e.g. class names, method names, etc.), defaults to false                                                                                                                    |
 | editor:wordwrap                      | bool     | set to true to enable word wrapping in the editor (defaults to false)                                                                                                                                                                                         |