docs/bpf: Add description of .BTF.base section

Now that .BTF.base sections are generated for out-of-tree kernel
modules (provided pahole supports the "distilled_base" BTF feature),
document .BTF.base and its role in supporting resilient split BTF
and BTF relocation.

Changes since v1:

- updated formatting, corrected typo, used BTF ID[s] consistently
  (Andrii)

Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20241028091543.2175967-1-alan.maguire@oracle.com