Multiple lines of text in single cell of simple table?

There is a clean way. The issue is by default the columns are set to no-wrap, so that's why you get the scroll. To fix that you have to override the css with the following:

/* override table no-wrap */
.wy-table-responsive table td, .wy-table-responsive table th {
    white-space: normal;
}

The simple table style does not support wrapping blocks. Use the grid style instead, like this:

+------------------+--------------+
| a short sentence | second cell  |
+------------------+--------------+
| a much longer    | bottom right |
| sentence         |              |
+------------------+--------------+

These tables are more tedious to work with, but they're more flexible. See the full documentation for details.

A workaround for this problem is to use a replace directive:

================  ============
a short sentence  second cell
|long_sentence|   bottom right
================  ============

.. |long_sentence| replace:: a much longer sentence