It appears that this issue, or something like it, has sprung up again: https://groups.google.com/forum/#!searchin/testng-users/Website$20javadoc%7Csort:relevance/testng-users/LFZWTLvkC_Q/2eYhUa3PAQAJ
The link from testng.org/doc to the JavaDocs has the URL https://jitpack.io/com/github/cbeust/testng/master-SNAPSHOT/javadoc/. Clicking that link merely gives the message "Not Found".
A friend of mine was able to find an internal page and forwarded it to me: https://jitpack.io/com/github/cbeust/testng/master-6.12-g412a3ba-117/javadoc/org/testng/Assert.html#assertEquals-double-double-double-
However, I found that even the internal links in this web page are mostly broken.