Update count methods docs

[ricardograca]

• Related Issues: count methods return String with Postgres #1275

Introduction

This updates the documentation for all count() methods. The biggest change is linking them all to the Model#count method that contains information about the return value type in PostgreSQL.

Motivation

It's a bit unexpected that the count() methods return strings in PostgreSQL, so this tries to help with that by making it more clear. There is also a solution in case the user would like to change the type parser to return numbers instead.

Fixes #1275.

References

• https://github.com/brianc/node-pg-types
• Add ability to opt-in to int8 parsing brianc/node-postgres#427
• 'bigint' type is a string in query results. knex/knex#387
• https://knexjs.org/#Builder-count