FIPS docs: clarify system FIPS mode vs. Go Runtime FIPS mode

[dagood]

(Followup item from initial 1.24 FIPS docs PRs.)

There's more nuance in 1.24 about FIPS modes. This can affect things like TLS fipsonly.

On the existing platforms (Linux, Windows), behavior matches what I'd expect based on earlier releases. However, the preview macOS support makes distinctions between platform FIPS, crypto library FIPS, and Go runtime FIPS more important to the user, because macOS doesn't seem to have a widely accepted "system FIPS mode" that we can refer to at runtime.