`expression` is a std::variant with 16 different variants
that represent different types of AST nodes.
Let's add documentation that explains what each of these
16 types represents. For people who are not familiar with expression
code it might not be clear what each of them does, so let's add
clear descriptions for all of them.
Signed-off-by: Jan Ciolek <jan.ciolek@scylladb.com>
Closesscylladb/scylladb#15767
c256cca6f1