Fulltext search docs improvement

[keerlu]

Currently we have the following docs for full text search/indexes:

• Indexes concepts page has a section on Full-text indexes for MySQL and MongoDB
• Full-text search concepts page for PostgreSQL and MySQL
• Client API reference entry forsearch for PostgreSQL and MySQL
• Filtering and sorting concepts page has a Sort by relevance section for PostgreSQL

Missing:

• documentation of @fulltext in the schema reference attributes section
• document index limitation for MongoDB - see prisma/prisma#2729
• MongoDB full text search?
• PostgreSQL full text indexes?
• links to index configuration docs from client full text search docs - see comment here

[janpio]

Current state:

MongoDB full text search? Not implemented yet.

PostgreSQL full text indexes? Not adequately implemented yet to be worth documenting.

[janpio]

Note that _relevance is related to fullTextSearch and has a docs issue as well: https://github.com/prisma/docs/issues/5045