The describe-mode function provides information about major
modes. It is normally bound to C-h m. It uses the value of the
variable major-mode (see Major Modes), which is why every
major mode command needs to set that variable.
This command displays the documentation of the current buffer’s major
mode and minor modes. It uses the documentation function to
retrieve the documentation strings of the major and minor mode
commands (see Access to Documentation Strings).
If called from Lisp with a non-nil buffer argument, this
function displays the documentation for that buffer’s major and minor
modes, rather than those of the current buffer.