Add comment about driver pgnative

Added a comment to the guide referencing the pgnative driver, similar
to the comment mentioning better-sqlite3. Fixes #539

Removed an unneccessary "if" from the PostgreSQL info block

src/guide/index.md

@@ -53,7 +53,7 @@ Knex's PostgreSQL client allows you to set the initial search path for each conn
 
 ```js
 const pg = require('knex')({
-  client: 'pg',
+  client: 'pg', // or 'pgnative'
   connection: process.env.PG_CONNECTION_STRING,
   searchPath: ['knex', 'public'],
 });
@@ -63,11 +63,11 @@ const pg = require('knex')({
 When using the PostgreSQL driver, another usage pattern for instantiating the Knex configuration object could be to use a `connection: {}` object details to specify various flags such as enabling SSL, a connection string, and individual connection configuration fields all in the same object. Consider the following example:
 
 ::: info PostgreSQL
-If `connectionString` is highest priority to use. If left unspecified then connection details will be determined using the individual connection fields (`host`, `port`, etc), and finally an SSL configuration will be enabled based on a truthy value of `config["DB_SSL"]` which will also accept self-signed certificates.
+`connectionString` is highest priority to use. If left unspecified then connection details will be determined using the individual connection fields (`host`, `port`, etc), and finally an SSL configuration will be enabled based on a truthy value of `config["DB_SSL"]` which will also accept self-signed certificates.
 
 ```js
 const pg = require('knex')({
-  client: 'pg',
+  client: 'pg', // or 'pgnative'
   connection: {
     connectionString: config.DATABASE_URL,
     host: config["DB_HOST"],