Update gui_mac.txt for MMShareFindPboard

Author: splhack

runtime/doc/gui_mac.txt

@@ -242,6 +242,7 @@ MMLoginShellArgument		login shell parameter [string]
 MMLoginShellCommand		which shell to use to launch Vim [string]
 MMNoFontSubstitution		disable automatic font substitution [bool]
 MMNoTitleBarWindow		hide title bar [bool]
+MMShareFindPboard		share search text to Find Pasteboard [bool]
 MMShowAddTabButton		enable "add tab" button on tabline [bool]
 MMTabMaxWidth			maximum width of a tab [int]
 MMTabMinWidth			minimum width of a tab [int]