Add important info about ANALYZE after create Functional Index

Hi all,

As you all already know Postgres supports functions in index expressions (marked as immutable ofc) and for this special index the ANALYZE command creates some statistics (new pg_statistic entry) about it.

The problem is just after creating a new index or rebuilding concurrently (using the new REINDEX .. CONCURRENTLY or the old manner creating new one and then swapping) we need to run ANALYZE to update statistics but we don't mention it in any part of our documentation.

Last weekend Gitlab went down because the lack of an ANALYZE after rebuilding concurrently a functional index and they followed the recommendation we have into our documentation [1] about how to rebuild it concurrently, but we don't warn users about the ANALYZE after.

Would be nice if add some information about it into our docs but not sure where. I'm thinking about:

- doc/src/sgml/ref/create_index.sgml

- doc/src/sgml/maintenance.sgml (routine-reindex)

Thoughts?

[1] https://gitlab.com/gitlab-com/gl-infra/production/-/issues/2885#note_436310499