[DOCS] case-insensitive index lower()

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/9.1/static/sql-createindex.html
Description:

In the section on indexes the documentation accurately describes how to
create an efficient, case-insensitive index using lower(field_name). But! in
Firebird to get a query to utilize such index, you must write the query as
such:

WHERE myfield = lower(:myfield)

Otherwise a table scan is performed. The documentation should state whether
or not such a thing is required. It's one thing to create an index, it's
quite another to know how to write a statement that guarantees that the
index will be used.