Update test exec platform documentation.

[gregestren]

Default behavior changed at 0dd0da9.

Also space out some paragraphs more to de-densify content.

[gregestren]

@fmeum I'm doing this in support of Bazel 9 release notes.

How would you say this fits with these relnotes from #24964 ?

RELNOTES[INC]: Constraints and toolchain requirements added to the default exec group, for example via the toolchains parameter of the rule function or the exec_compatible_with attribute on all rules, no longer apply to the test exec group, which contains the test action on test rules. Instead, use exec_group_compatible_with to apply constraints and/or define an explicit test exec group with toolchain requirements on test rules that require it.

I think that's still relevant, but a bit more nuanced since tests now have different default toolchain semantics?

[fmeum]

Thanks! The other relnotes seem worth keeping if we also mention the flag flip for the default test toolchain.