docs/btf: fix typos, improve wording

Fix various typos, some of the formatting and wording for
Documentation/btf.rst.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>