A bulk of the Vulkan 1.0.40 changes come from switching from the Python-based asciidoc to using the Ruby-written asciidoctor, for generating the documentation.
Vulkan 1.0.40 also has updates to various extension documentation, some minor clarifications, a small update to GL_KHR_vulkan_glsl, and various other work.
There is a new extension and it's VK_EXT_SMPTE2086_metadata. But VK_EXT_SMPTE2086_metadata isn't particularly interesting to most Vulkan users at the moment, "This extension defines two new structures and a function to assign SMPTE (the Society of Motion Picture and Television Engineers) 2086 metadata to a swapchain. This is the Vulkan equivalent to the EGL_EXT_surface_SMPTE2086_metadata extension."
SMPTE 2086 is used for passing HDR-related meta-data between the GPU and monitor. So this will be good for Vulkan HDR rendering on HDR displays, but right now for Linux users there is more Linux HDR work to happen to improve the ecosystem, not to mention most Linux gamers / desktop users don't yet have High Dynamic Range displays.
Vulkan 1.0.40 details via GitHub.