If you're worried about learning some new programming language, you can bet the one with a better documentation is the one that is born with a document processor.

Same goes for the frameworks/libraries of that language.

The answer for that is the same as languages that come with tests: because the programming language standard library comes with a documentation generator or even because documentation is bundled in the language itself, it reduces the friction needed to start writing the documentation.

Python is a curious case that it came with a simple documentation generator (PyDoc) and a bundled documentation format (DocStrings). Nowadays, almost nobody is using the default documentation generator anymore, but because the documentation format is still there and is still supported by the language (documentation appears as a property of every function, class and module), other tools took the post of default documentation generator, but the documentation format is still heavy used.

Also, the opposite seems almost always true: If the language doesn't come with integrated documentation, there is a very good chance that the documentation or the language or frameworks and libraries will be bad. Or, in the very least, every library will pick its own format, every framework will pick its own format and they will never match the language format, and you'll end up with a mess of a documentation to decipher.