Add some use-case information to PseudoTerminal#indexOfTab

Headers/iTerm/PseudoTerminal.h

@@ -250,6 +250,9 @@ typedef enum {
 - (void)selectSessionAtIndexAction:(id)sender;
 // Return the index of a tab or NSNotFound.
+// This method is used, for example, in iTermExpose, where PTYTabs are shown
+// side by side, and one needs to determine which index it has, so it can be
+// selected when leaving iTerm expose.
 - (NSInteger)indexOfTab:(PTYTab*)aTab;
 // Open a new tab with the bookmark given by the guid in