The community has developed and evolved the Linux kernel documentation into a good state over many years of effort. The latest version of the kernel documentation, presented in a nice and modern "web" style, can always be accessed online here: https://www.kernel.org/doc/html/latest/.
Of course, as we will mention in the next chapter, the kernel documentation is always available for that kernel version within the kernel source tree itself, in the directory called Documentation/.
As just one example of the online kernel documentation, see the following partial screenshot of the page on Core Kernel Documentation/Basic C Library Functions (https://www.kernel.org/doc/html/latest/core-api/kernel-api.html#basic-c-library-functions):
Figure 1.8 – Partial screenshot showing a small part of the modern online Linux kernel documentation
As can be gleaned from the screenshot, the modern documentation is pretty comprehensive.