kernel-doc: Update "cross-referencing from rST" section to use automarkup

Cross-referencing from reStructuredText

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

To cross-reference the functions and types defined in the kernel-doc comments

from reStructuredText documents, please use the `Sphinx C Domain`_

references. For example::

  See function :c:func:`foo` and struct/union/enum/typedef :c:type:`bar`.

While the type reference works with just the type name, without the

struct/union/enum/typedef part in front, you may want to use::

  See :c:type:`struct foo <foo>`.

  See :c:type:`union bar <bar>`.

  See :c:type:`enum baz <baz>`.

  See :c:type:`typedef meh <meh>`.

This will produce prettier links, and is in line with how kernel-doc does the

cross-references.

No additional syntax is needed to cross-reference the functions and types

defined in the kernel-doc comments from reStructuredText documents.

Just end function names with ``()`` and write ``struct``, ``union``, ``enum``

or ``typedef`` before types.

For example::

  See foo().

  See struct foo.

  See union bar.

  See enum baz.

  See typedef meh.

However, if you want custom text in the cross-reference link, that can be done

through the following syntax::

  See :c:func:`my custom link text for function foo <foo>`.

  See :c:type:`my custom link text for struct bar <bar>`.

For further details, please refer to the `Sphinx C Domain`_ documentation.