The reStructuredText format (also known as RST, ReST, or reST) was developed in 2002 as a simple language that implements enough markup to be usable, but is simple enough to be readable as plain text. These two features make it readable enough to use in code, yet still versatile enough to generate pretty and useful documentation.
The greatest thing about reStructuredText is that it is very intuitive. Even without knowing anything about the standard, you can easily write documentation in this style without ever knowing that it would be recognized as a language. However, more advanced techniques, such as images and links, do require some explanation.
Next to reStructuredText, there are also languages such as Markdown which are quite similar in usage. Within the Python community, reStructuredText has been the standard documentation language for over 10 years, making it the recommended solution.