[ticket/12397] Fix sql_unique_index_exists doc block

db_tools::sql_unique_index_exists() searches for unique indexes but not
primary key indexes.

PHPBB3-12397
Merge pull request #2310 from n-aleha/ticket/12397

[ticket/12397] Fix sql_unique_index_exists doc block

* n-aleha/ticket/12397:
  [ticket/12397] Fix sql_unique_index_exists doc block