doc: fix default symbol binding in ABI guide

doc: fix default symbol binding in ABI guide

The document abi_versioning.rst incorrectly instructs the developer to
add BIND_DEFAULT_SYMBOL to the public header, not the source file. This
commit fixes the issue and adds some clarifications.

The commit also clarifies the use of use_function_versioning in the
meson/ninja build system, and does some minor re-organization of the
document.

Fixes: f1ef9794f9bd ("doc: add ABI guidelines")
Cc: stable@dpdk.org
Signed-off-by: Ray Kinsella <mdr@ashroe.eu>