clarify what session provider 'db' does (#19713)

* clarify what session provider 'db' does

* Also update session.PROVIDER in app.example.ini

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

custom/conf/app.example.ini

@@ -1608,7 +1608,8 @@ PATH =
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;;
-;; Either "memory", "file", or "redis", default is "memory"
+;; Either "memory", "file", "redis", "db", "mysql", "couchbase", "memcache" or "postgres"
+;; Default is "memory". "db" will reuse the configuration in [database]
 ;PROVIDER = memory
 ;;
 ;; Provider config options

docs/content/doc/advanced/config-cheat-sheet.en-us.md

@@ -697,7 +697,7 @@ Define allowed algorithms and their minimum key length (use -1 to disable a type
 
 ## Session (`session`)
 
-- `PROVIDER`: **memory**: Session engine provider \[memory, file, redis, db, mysql, couchbase, memcache, postgres\].
+- `PROVIDER`: **memory**: Session engine provider \[memory, file, redis, db, mysql, couchbase, memcache, postgres\]. Setting `db` will reuse the configuration in `[database]`
 - `PROVIDER_CONFIG`: **data/sessions**: For file, the root path; for db, empty (database config will be used); for others, the connection string.
 - `COOKIE_SECURE`: **false**: Enable this to force using HTTPS for all session access.
 - `COOKIE_NAME`: **i\_like\_gitea**: The name of the cookie used for the session ID.