docs: sysctl/kernel: document firmware_config

Based on the firmware fallback mechanisms documentation and the
implementation in drivers/base/firmware_loader/fallback.c.
Signed-off-by: Stephen Kitt <steve@sk2.org>
Link: https://lore.kernel.org/r/20200429205757.8677-2-steve@sk2.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/sysctl/kernel.rst

@@ -265,6 +265,19 @@ domain names are in general different. For a detailed discussion
 see the ``hostname(1)`` man page.
 
 
+firmware_config
+===============
+
+See :doc:`/driver-api/firmware/fallback-mechanisms`.
+
+The entries in this directory allow the firmware loader helper
+fallback to be controlled:
+
+* ``force_sysfs_fallback``, when set to 1, forces the use of the
+  fallback;
+* ``ignore_sysfs_fallback``, when set to 1, ignores any fallback.
+
+
 ftrace_dump_on_oops
 ===================