bpf, docs: Fix invalid escape sequence warnings in bpf_doc.py

The script bpf_doc.py generates multiple SyntaxWarnings related to invalid
escape sequences when executed with Python 3.12. These warnings do not appear
in Python 3.10 and 3.11 and do not affect the kernel build, which completes
successfully.

This patch resolves these SyntaxWarnings by converting the relevant string
literals to raw strings or by escaping backslashes. This ensures that
backslashes are interpreted as literal characters, eliminating the warnings.

Reported-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Signed-off-by: Vishal Chourasia <vishalc@linux.ibm.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/20230829074931.25112048-1-vishalc@linux.ibm.com