docs: submitting-patches: use :doc: for references

There are two broken references at submitting-patches.rst:

	Documentation/process/submitting-patches.rst:240: WARNING: undefined label: security-bugs (if the link has no caption the label must precede a section header)
	Documentation/process/submitting-patches.rst:336: WARNING: undefined label: documentation/process/email-clients.rst (if the link has no caption the label must precede a section header)

Those are due to some recent renames and file moves.

It turns that maintaining :ref: is currently harder than using
:doc:, as we now have a script to help checking such references.

So, replace :ref: to :doc: there, making them to point to the
current file name.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/3ba405f579cf35ef2b39dd210d8ad46adc79f0ad.1599660067.git.mchehab+huawei@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>