docs(cc): clarify abi_tag is equivalent to SOABI and cite PEP 3149

[rickeylev]

Clarify the documentation for abi_tag in C++ Python toolchains so
that rule maintainers and users can easily map Bazel's toolchain
attributes to standard Python distribution environment variables.

Update docstrings in python/private/py_cc_toolchain_info.bzl and
python/private/py_cc_toolchain_rule.bzl to explicitly note that
abi_tag is the equivalent of the Python sysconfig variable SOABI,
accompanied by a reference citation to PEP 3149.

Also add :::{versionadded} VERSION_NEXT_FEATURE directives to all newly
introduced toolchain fields and rule attributes.