Adding descriptions for the missing attributes: 'valueBlobFile', 'valueClobFile' and 'encoding'.

Descriptions for [BLOB/CLOB feature](https://liquibase.jira.com/browse/CORE-1460) taking into account changes made in [this commit] (https://github.com/willianconsta/liquibase/commit/f73730874be5e8f7f842b5f8bb0caca48b3a30d1) (columns were renamed).