Document algorithm for record:rand() generations (#889)

src/content/doc-surrealql/datamodel/ids.mdx

@@ -90,6 +90,8 @@ Record IDs can be generated with a number of built-in ID generation functions. T
 
 ```surql
 // Generate a random record ID
+// Charset: `abcdefghijklmnopqrstuvwxyz0123456789`
+// ID Length: 20 characters long
 CREATE temperature:rand() SET time = time::now(), celsius = 37.5;
 // Generate a ULID-based record ID
 CREATE temperature:ulid() SET time = time::now(), celsius = 37.5;
@@ -209,4 +211,4 @@ FROM { temps: [-5, 8, 9] };
 
 ## Learn more 
 
-Learn more about Record IDs [in this blogpost](https://surrealdb.com/blog/the-life-changing-magic-of-surrealdb-record-ids#the-performance-at-scale) and on this [youtube video](https://www.youtube.com/watch?v=c0cqmWRYP8c).
+Learn more about Record IDs [in this blogpost](https://surrealdb.com/blog/the-life-changing-magic-of-surrealdb-record-ids#the-performance-at-scale) and on this [youtube video](https://www.youtube.com/watch?v=c0cqmWRYP8c).