Fix escaping of documentation. Escape final string output

I tried to generate documentation for downstream Euslisp files such as ``pr2eus/robot-interface.l`` or ``hrpsys_ros_bridge/euslisp/rtm-ros-robot-interface.l``:
```
roseus ../eus/lib/llib/documentation.l "(make-document (ros::resolve-ros-path \"package://hrpsys_ros_bridge/euslisp/rtm-ros-robot-interface.l\") \"/tmp/rtm-ros-robot-interface-func.tex\")" "(exit)"
```

Generated tex files contain "_" without escaping.
So, i think we need to escape final output strings and created this PR.
Please check this commit.