docs: i2c: avoid using ReST :doc:`foo` markup

The :doc:`foo` tag is auto-generated via automarkup.py.
So, use the filename at the sources, instead of :doc:`foo`.

Acked-by: Wolfram Sang <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>
Link: https://lore.kernel.org/r/569722[e-mail]
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/i2c/instantiating-devices.rst

···
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 ACPI can also describe I2C devices. There is special documentation for this
-which is currently located at :doc:`../firmware-guide/acpi/enumeration`.
+which is currently located at Documentation/firmware-guide/acpi/enumeration.rst.
 
 
 Declare the I2C devices in board files

Documentation/i2c/old-module-parameters.rst

···
 With the conversion of the I2C subsystem to the standard device driver
 binding model, it became clear that these per-module parameters were no
 longer needed, and that a centralized implementation was possible. The new,
-sysfs-based interface is described in :doc:`instantiating-devices`, section
+sysfs-based interface is described in
+Documentation/i2c/instantiating-devices.rst, section
 "Method 4: Instantiate from user-space".
 
 Below is a mapping from the old module parameters to the new interface.

Documentation/i2c/smbus-protocol.rst

···
 Each transaction type corresponds to a functionality flag. Before calling a
 transaction function, a device driver should always check (just once) for
 the corresponding functionality flag to ensure that the underlying I2C
-adapter supports the transaction in question. See :doc:`functionality` for
-the details.
+adapter supports the transaction in question. See
+Documentation/i2c/functionality.rst for the details.
 
 
 Key to symbols