Display documentation of current major mode and minor modes.
A brief summary of the minor modes comes first, followed by the
major mode description. This is followed by detailed
descriptions of the minor modes, each on a separate page.
For this to work correctly for a minor mode, the mode's indicator
variable (listed in `minor-mode-alist') must also be a function
whose documentation describes the minor mode.
If called from Lisp with a non-nil BUFFER argument, display
documentation for the major and minor modes of that buffer.