[record_use] Document compiler differences

[dcharkes]

Compilers can have small differences:

• [record_use] Correctness: Non-obvious const argument values (differences between backends) #2889

This PR documents some possible differences and how they show up.

A follow up PR should add this kind of docs to a logical place in the Dartdoc (maybe the library dartdoc and the readme).

Another follow up PR should add a semanticEquals method so we can test that the output of two compilers is semantically identical:

• [record_use] Implementation: Semantic equality for RecordedUses for testing #2897