message: Add help text for "expression" telling how to enter multi-line mode.
  This seemed natural to us, but wasn't documented anywhere and was
  not clear to everybody.
  <rdar://problem/33316164>
diff: @@ -228,6 +228,16 @@ CommandObjectExpression::CommandObjectExpression(
  m_command_options(), m_expr_line_count(0), m_expr_lines() {
  SetHelpLong(
  R"(
  +Single and multi-line expressions:
  +
  +)"
  + " The expression provided on the command line must be a complete expression \
  +with no newlines. To evaluate a multi-line expression, \
  +hit a return after an empty expression, and lldb will enter the multi-line expression editor. \
  +Hit return on an empty line to end the multi-line expression."
  +
  + R"(
  +
  Timeouts:
  )"
  @@ -256,6 +266,7 @@ from the stack with \"thread return -x\" or if you are still interested in the e
  you can issue the \"continue\" command and the expression evaluation will complete and the \
  expression result will be available using the \"thread.completed-expression\" key in the thread \
  format."
  +
  R"(
  Examples:
  