In the format overview documentation, as seen at https://www.xiph.org/flac/documentation_format_overview.html, in the inter-channel decorrelation section, it currently says "This is a lossless process, unlike joint stereo."
I think it would be ideal to remove the qualifier and just say "This is a lossless process."
It propagates a myth that joint stereo is necessarily lossy.
The term joint stereo is mainly used in the MPEG audio specs, and it covers several coding methods: intensity stereo (IS) and mid-side (MS) stereo being the main ones, as alternatives to the default of simple (SS or LR) stereo.
IS certainly is lossy and is the only joint stereo option in MP1 & MP2. MP3 & AAC add the option of using MS or SS on a per-frame basis—i.e., a "joint stereo" frame may well be an ordinary SS frame with a different flag in the header. So, joint stereo isn't necessarily lossy, per se. However, the choice of MS or SS can affect quality in a lossy encoder. Which one is better depends on the correlation of the input channels and how much bandwidth is allocated to the channels in the frame.
This is too much to get into, though. Just avoid the term "joint stereo" entirely.