[DOCS] Excluding common values from the index

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/9.6/static/indexes-partial.html
Description:

Rather than a report of an error, this letter is a suggestion of improvement
of PostgreSQL.

As per page https://www.postgresql.org/docs/9.6/static/indexes-partial.html,
PostgreSQL avoids using indices when retrieving common values.

Then, maybe, during the construction of an index, PostgreSQL should be
detecting such common values, and exclude them from the index automatically.