docs: clarify parameter naming requirements
Fixes
@@ -29,8 +29,8 @@ import (
// A Statement is a SQL query with named parameters.
//
// A parameter placeholder consists of '@' followed by the parameter name.
-// Parameter names consist of any combination of letters, numbers, and
-// underscores. Names may be entirely numeric (e.g., "WHERE m.id = @5").
+// The parameter name is an identifier which must conform to the naming
+// requirements in https://cloud.google.com/spanner/docs/lexical#identifiers.
// Parameters may appear anywhere that a literal value is expected. The same
// parameter name may be used more than once. It is an error to execute a
// statement with unbound parameters. On the other hand, it is allowable to
