Fix documentation around optional arguments. This includes:

- Syncing between the usage string in code and in the man page.
- Adding optional arguments that were not mentioned (such as
  shell-command arguments).
- Adding square brackets around arguments that are actually optional.

From Julian Prein (julian at druck dot dev) in GitHub issue 4419.

cmd-split-window.c

@@ -43,7 +43,7 @@ const struct cmd_entry cmd_split_window_entry = {
 	.args = { "bc:de:fF:hIl:p:Pt:vZ", 0, -1, NULL },
 	.usage = "[-bdefhIPvZ] [-c start-directory] [-e environment] "
 		 "[-F format] [-l size] " CMD_TARGET_PANE_USAGE
-		 " [shell-command]",
+		 " [shell-command [argument ...]]",
 
 	.target = { 't', CMD_FIND_PANE, 0 },