Documentation: bpf: Escape underscore in BPF type name prefix

Sphinx reported unknown target warning:

Documentation/bpf/bpf_design_QA.rst:329: WARNING: Unknown target name: "bpf".

The warning is caused by BPF type name prefix ("bpf_") which is written
without escaping the trailing underscore.

Escape the underscore to fix the warning. While at it, wrap the
containing paragraph in less than 80 characters.

Fixes: 9805af8d ("bpf: Document UAPI details for special BPF types")
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: KP Singh <kpsingh@kernel.org>
Acked-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/bpf/20221104123913.50610-1-bagasdotme@gmail.com