Add comment with example strings matched or not by regex

As suggested in code review.

Author: thomasspriggs

src/solvers/smt2_incremental/smt_terms.cpp

@@ -50,6 +50,8 @@ bool smt_bool_literal_termt::value() const
 
 static bool is_valid_smt_identifier(irep_idt identifier)
 {
+  // The below regex matches a complete string which does not contain the `|`
+  // character. So it would match the string `foo bar`, but not `|foo bar|`.
   static const std::regex valid{R"(^[^\|]*$)"};
   return std::regex_match(id2string(identifier), valid);
 }