libbpf: Add documentation to version and error API functions

Add documentation for the following API functions:

- libbpf_major_version()
- libbpf_minor_version()
- libbpf_version_string()
- libbpf_strerror()

Signed-off-by: Cryolitia PukNgae <cryolitia@uniontech.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20250820-libbpf-doc-1-v1-1-13841f25a134@uniontech.com