fix #229: update doc on Where to avoid confusion

Author: huandu

delete.go

@@ -125,7 +125,10 @@ func (db *DeleteBuilder) TableNames() []string {
 	return tableNames
 }
 
-// Where sets expressions of WHERE in DELETE.
+// Where adds expressions to the WHERE clause in DELETE.
+//
+// Multiple calls to Where will join expressions with AND.
+// To reset the WHERE clause, set the WhereClause field to nil.
 func (db *DeleteBuilder) Where(andExpr ...string) *DeleteBuilder {
 	if len(andExpr) == 0 || estimateStringsBytes(andExpr) == 0 {
 		return db

select.go

@@ -205,7 +205,10 @@ func (sb *SelectBuilder) JoinWithOption(option JoinOption, table string, onExpr
 	return sb
 }
 
-// Where sets expressions of WHERE in SELECT.
+// Where adds expressions to the WHERE clause in SELECT.
+//
+// Multiple calls to Where will join expressions with AND.
+// To reset the WHERE clause, set the WhereClause field to nil.
 func (sb *SelectBuilder) Where(andExpr ...string) *SelectBuilder {
 	if len(andExpr) == 0 || estimateStringsBytes(andExpr) == 0 {
 		return sb

update.go

@@ -140,7 +140,10 @@ func (ub *UpdateBuilder) SetMore(assignment ...string) *UpdateBuilder {
 	return ub
 }
 
-// Where sets expressions of WHERE in UPDATE.
+// Where adds expressions to the WHERE clause in UPDATE.
+//
+// Multiple calls to Where will join expressions with AND.
+// To reset the WHERE clause, set the WhereClause field to nil.
 func (ub *UpdateBuilder) Where(andExpr ...string) *UpdateBuilder {
 	if len(andExpr) == 0 || estimateStringsBytes(andExpr) == 0 {
 		return ub