Optionally recommend against static import of Truth8.assertThat.

[copybara-service]

Optionally recommend against static import of Truth8.assertThat.

The methods there will be moving into the main Truth class. Users will have various options for how to migrate, but one option involves avoiding static imports. We will provide more instructions as part of our Truth releases. (See, e.g., the release notes for Truth 1.3.0.)

Rough outline of the option that involves avoiding static imports:

• The problem with adding Truth8.assertThat(OptionalInt) (or a similar overload) to Truth is that anyone who static imports both Truth.assertThat and Truth8.assertThat will see ambiguity errors if they use that method.
• The solution is to stop using static import for Truth8.assertThat. Once all its static imports are gone, we can add the new methods to Truth without breaking anyone, and then we can migrate users.
• Avoiding static import for Truth8.assertThat would be tough to swallow for the most common types, Stream and Optional. Luckily, I found an alternative approach that works for those, as discussed in the linked release notes. (That approach works for those types because they're generic types.)