Improve NGINX integration doc, clarify hostname/URL envvars

[djwfyi]

The NGINX integration doc needs some tweaks to clarify how to get things going.

Clarify the purpose and use of the following envvar and document:

• MINIO_SERVER_URL
• MINIO_BROWSER_REDIRECT_URL
• MINIO_DOMAIN

Clarify the purpose and use of the following NGINX properties for MinIO

• location
• proxy_pass

Update the following pages as needed to reflect their use:

• https://min.io/docs/minio/linux/integrations/setup-nginx-proxy-with-minio.html
• https://min.io/docs/minio/linux/reference/minio-server/minio-server.html#core-configuration

[djwfyi]

See minio/console#2818 (comment)

[ravindk89]

Minor confirmation @harshavardhana

• MINIO_SERVER_URL sets the FQDN MinIO uses for any operation that requires returning a hostname. This is important for MinIO Server under a reverse proxy to ensure MinIO uses that proxy address in responses vs an internal hostname or IP

• MINIO_BROWSER_REDIRECT_URL sets the FQDN MinIO Console uses for any operation that requires returning a hostname. Same basic logic.

If you specify MINIO_SERVER_URL only, the Console may not respond to certain requests (e.g. presigned URL) using the correct hostname when behind a reverse proxy.