Add comments to the test files

tests/sqlparser_common.rs

@@ -1,4 +1,10 @@
 #![warn(clippy::all)]
+//! Test SQL syntax, which all sqlparser dialects must parse in the same way.
+//!
+//! Note that it does not mean all SQL here is valid in all the dialects, only
+//! that 1) it's either standard or widely supported and 2) it can be parsed by
+//! sqlparser regardless of the chosen dialect (i.e. it doesn't conflict with
+//! dialect-specific parsing rules).
 
 use matches::assert_matches;
 

tests/sqlparser_mssql.rs

@@ -1,4 +1,6 @@
 #![warn(clippy::all)]
+//! Test SQL syntax specific to Microsoft's T-SQL. The parser based on the
+//! generic dialect is also tested (on the inputs it can handle).
 
 use sqlparser::dialect::{GenericSqlDialect, MsSqlDialect};
 use sqlparser::sqlast::*;

tests/sqlparser_postgres.rs

@@ -1,4 +1,6 @@
 #![warn(clippy::all)]
+//! Test SQL syntax specific to PostgreSQL. The parser based on the
+//! generic dialect is also tested (on the inputs it can handle).
 
 use sqlparser::dialect::{GenericSqlDialect, PostgreSqlDialect};
 use sqlparser::sqlast::*;