Fix and clarify OrWhere caveats in SqlBuilder docs

Dapper.SqlBuilder/Readme.md

@@ -85,10 +85,16 @@ var count = conn.ExecuteScalar<int>(countTemplate.RawSql, countTemplate.Paramete
 Limitations and caveats
 --------
 
-OrWhere use `and` not `or` to concat sql problem
+### Combining the Where and OrWhere methods
 
-[Issue 647](https://github.com/DapperLib/Dapper/issues/647) 
+The OrWhere method currently groups all `and` and `or` clauses by type,
+then join the groups with `and` or `or` depending on the first call.
+This may result in possibly unexpected outcomes.
+See also [issue 647](https://github.com/DapperLib/Dapper/issues/647).
 
+#### Example Where first
+
+When providing the following clauses
 ```csharp
 sql.Where("a = @a1");
 sql.OrWhere("b = @b1");
@@ -97,11 +103,26 @@ sql.OrWhere("b = @b2");
 ```
 
 SqlBuilder will generate sql
-```sql=
-a = @a1 AND b = @b1 AND a = @a2 AND b = @b2
+```sql
+a = @a1 AND a = @a2 AND ( b = @b1 OR b = @b2 )
 ```
 
-not
+and not say
 ```sql
 a = @a1 OR b = @b1 AND a = @a2 OR b = @b2
 ```
+
+#### Example OrWhere first
+
+When providing the following clauses
+```csharp
+sql.OrWhere("b = @b1");
+sql.Where("a = @a1");
+sql.OrWhere("b = @b2");
+sql.Where("a = @a2");
+```
+
+SqlBuilder will generate sql
+```sql
+a = @a1 OR a = @a2 OR ( b = @b1 OR b = @b2 )
+```