captive python instructions need to be updated

[bendavini]

With the introduction of the module-based dependency on rules_python (https://github.com/salesforce/rules_spring/pull/190), the python toolchain from rules_python is the default, even if users want to use another (e.g., a 'captive' python).

Following the existing instructions to use a 'captive' python are insufficient; users must now either:

• specify the preferred toolchain at the command line, e.g. with --extra_toolchains //tools/python_interpreter:captive_python_toolchain or
• change is_default = True to is_default = False in MODULE.bazel for the python.toolchain
• ???

[plaird]

@bendavini researched the issue and provided doc updates for this, which have been merged.

[plaird]

We are finally getting closer to migrate to bzl mod ourselves. Nice to have some of these issues already sorted out!