doc/md: add docs on using expression predicates in custom WHERE clauses (#2520)

* Add docs on using ExprP() for custom WHERE sql statement

* Add ExprP() examples using integration test

* move custom predicate example and show examples using both P() and ExprP()

* fix to greater or equal than

* rephrase wording and fix sql builder

* Update doc/md/predicates.md

Co-authored-by: Ariel Mashraki <7413593+a8m@users.noreply.github.com>

* Update doc/md/predicates.md

* Update doc/md/predicates.md

* Update doc/md/predicates.md

* Update doc/md/predicates.md

Co-authored-by: Ariel Mashraki <7413593+a8m@users.noreply.github.com>

doc/md/predicates.md

@@ -89,6 +89,41 @@ client.Pet.
 
 Custom predicates can be useful if you want to write your own dialect-specific logic or to control the executed queries.
 
+For example, in order to use built-in SQL functions such as `DATE()`, use one of the following options:
+
+1. Pass a dialect-aware predicate function using the `sql.P` option:
+
+```go
+users := client.User.Query().
+	Select(user.FieldID).
+	Where(sql.P(func(b *sql.Builder) {
+		b.WriteString("DATE(").Ident("last_login_at").WriteByte(')').WriteOp(OpGTE).Arg(value)
+	})).
+	AllX(ctx)
+```
+
+The above code will produce the following SQL query:
+
+```sql
+SELECT `id` FROM `users` WHERE DATE(`last_login_at`) >= ?
+```
+
+2. Inline a predicate expression using the `ExprP()` option:
+
+```go
+users := client.User.Query().
+	Select(user.FieldID).
+	Where(func(s *sql.Selector) {
+		s.Where(sql.ExprP("DATE(last_login_at >= ?", value))
+	}).
+	AllX(ctx)
+```
+
+The above code will produce the same SQL query:
+
+```sql
+SELECT `id` FROM `users` WHERE DATE(`last_login_at`) >= ?
+```
 #### Get all pets of users 1, 2 and 3
 
 ```go