fix for potential read builtin crash if bash gets a signal before read reads any input; make wait -f work if there are no pid or job arguments and update documentation; save and restore variable reflected in $BASH_COMMAND around shell function calls

doc/bashref.texi

@@ -10198,6 +10198,8 @@ This is useful only when used with the @option{-n} option.
 Supplying the @option{-f} option, when job control is enabled,
 forces @code{wait} to wait for each @var{id} to terminate before
 returning its status, instead of returning when it changes status.
+If there are no @var{id} arguments,
+@code{wait} waits until all background processes have terminated.
 
 If none of the @var{id}s specify one of the shell's an active child
 processes, the return status is 127.