docs: Update docs + example.env for DB_PASSWORD (#11678)

docker/example.env

@@ -12,6 +12,7 @@ DB_DATA_LOCATION=./postgres
 IMMICH_VERSION=release
 
 # Connection secret for postgres. You should change it to a random password
+# Please use only the characters `A-Za-z0-9`, without special characters or spaces
 DB_PASSWORD=postgres
 
 # The values below this line do not need to be changed