add alter table add field example (#939)

Author: leto-b

src/UserGuide/Master/Table/Basic-Concept/Table-Management.md

@@ -280,6 +280,7 @@ ALTER TABLE (IF EXISTS)? tableName=qualifiedName ADD COLUMN (IF NOT EXISTS)? col
 
 ```SQL
 ALTER TABLE tableB ADD COLUMN IF NOT EXISTS a TAG
+ALTER TABLE tableB ADD COLUMN IF NOT EXISTS b FLOAT FIELD COMMENT 'b'
 ALTER TABLE tableB set properties TTL=3600
 COMMENT ON TABLE table1 IS 'table1'
 COMMENT ON COLUMN table1.a IS null

src/UserGuide/latest-Table/Basic-Concept/Table-Management.md

@@ -281,6 +281,7 @@ ALTER TABLE (IF EXISTS)? tableName=qualifiedName ADD COLUMN (IF NOT EXISTS)? col
 
 ```SQL
 ALTER TABLE tableB ADD COLUMN IF NOT EXISTS a TAG
+ALTER TABLE tableB ADD COLUMN IF NOT EXISTS b FLOAT FIELD COMMENT 'b'
 ALTER TABLE tableB set properties TTL=3600
 COMMENT ON TABLE table1 IS 'table1'
 COMMENT ON COLUMN table1.a IS null

src/zh/UserGuide/Master/Table/Basic-Concept/Table-Management.md

@@ -287,6 +287,7 @@ ALTER TABLE (IF EXISTS)? tableName=qualifiedName ADD COLUMN (IF NOT EXISTS)? col
 
 ```SQL
 ALTER TABLE tableB ADD COLUMN IF NOT EXISTS a TAG COMMENT 'a'
+ALTER TABLE tableB ADD COLUMN IF NOT EXISTS b FLOAT FIELD COMMENT 'b'
 ALTER TABLE tableB set properties TTL=3600
 COMMENT ON TABLE table1 IS 'table1'
 COMMENT ON COLUMN table1.a IS null

src/zh/UserGuide/latest-Table/Basic-Concept/Table-Management.md

@@ -287,6 +287,7 @@ ALTER TABLE (IF EXISTS)? tableName=qualifiedName ADD COLUMN (IF NOT EXISTS)? col
 
 ```SQL
 ALTER TABLE tableB ADD COLUMN IF NOT EXISTS a TAG COMMENT 'a'
+ALTER TABLE tableB ADD COLUMN IF NOT EXISTS b FLOAT FIELD COMMENT 'b'
 ALTER TABLE tableB set properties TTL=3600
 COMMENT ON TABLE table1 IS 'table1'
 COMMENT ON COLUMN table1.a IS null