docs: kdoc: Fix pdfdocs build for tools

the "\1" inside a docstring requires proper scaping to not be
considered a hex character and break the build.

Reported-by: Akira Yokosawa <akiyks@gmail.com>
Closes: https://lore.kernel.org/linux-doc/63e99049-cc72-4156-83af-414fdde34312@gmail.com/
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <2fff8ef1d0d64e8b68f15f5c07613f302d773855.1769500383.git.mchehab+huawei@kernel.org>

Author: mchehab

tools/lib/python/kdoc/kdoc_re.py

@@ -228,14 +228,18 @@ def search(self, regex, line):
             yield line[t[0]:t[2]]
 
     def sub(self, regex, sub, line, count=0):
-        """
+        r"""
         This is similar to re.sub:
 
         It matches a regex that it is followed by a delimiter,
         replacing occurrences only if all delimiters are paired.
 
-        if r'\1' is used, it works just like re: it places there the
-        matched paired data with the delimiter stripped.
+        if the sub argument contains::
+
+            r'\1'
+
+        it will work just like re: it places there the matched paired data
+        with the delimiter stripped.
 
         If count is different than zero, it will replace at most count
         items.