Update DB requirements in the documentation

This makes it more obvious that we greatly prefer PostgreSQL over
MySQL/MariaDB and that all users are recommended to use PostgreSQL
instead. This also removes the paragraph mentioning the size per user
row, as this is completely inaccurate and doesn't really help users with
anticipating the database size. Instead we now simply recommend users to
have at least 5-10 GB of storage at minimum.

[ci skip]

doc/install/requirements.md

@@ -122,15 +122,22 @@ To change the Unicorn workers when you have the Omnibus package please see [the
 
 We currently support the following databases:
 
-- PostgreSQL (recommended)
+- PostgreSQL
 - MySQL/MariaDB
 
-If you want to run the database separately, expect a size of about 1 MB per user.
+We _highly_ recommend the use of PostgreSQL instead of MySQL/MariaDB as not all
+features of GitLab may work with MySQL/MariaDB. Existing users using GitLab with
+MySQL/MariaDB are advised to migrate to PostgreSQL instead.
+
+The server running the database should have _at least_ 5-10 GB of storage
+available, though the exact requirements depend on the size of the GitLab
+installation (e.g. the number of users, projects, etc).
 
 ### PostgreSQL Requirements
 
-As of GitLab 9.0, PostgreSQL 9.6 is recommended. Lower versions of PostgreSQL
-may work but primary testing and developement takes place using PostgreSQL 9.6.
+As of GitLab 9.0, PostgreSQL 9.2 or newer is required, and earlier versions are
+not supported. We highly recommend users to use at least PostgreSQL 9.6 as this
+is the PostgreSQL version used for development and testing.
 
 Users using PostgreSQL must ensure the `pg_trgm` extension is loaded into every
 GitLab database. This extension can be enabled (using a PostgreSQL super user)