docs: conf.py: change the Sphinx 3.x+ text

After the build fix patches, it is believed that the output
with Sphinx 3.x won't be a problem. Still, the C domain
support was re-written, and this can have caused hidden
issues.

So, let's keep the warning, changing it to a lighter
warning text.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

Documentation/conf.py

@@ -47,8 +47,8 @@ extensions = ['kerneldoc', 'rstFlatTable', 'kernel_include',
 #
 if major >= 3:
     sys.stderr.write('''WARNING: The kernel documentation build process
-        does not work correctly with Sphinx v3.0 and above.  Expect errors
-        in the generated output.
+        support for Sphinx v3.0 and above is brand new. Be prepared for
+        possible issues in the generated output.
         ''')
     if minor > 0 or patch >= 2:
         # Sphinx c function parser is more pedantic with regards to type