docs: enhance tagging filter documentation

[qfritz]

This will allow a proper release of the added tag filter in #232 by @msvticket .

[github-actions]

Package	Line Rate	Health
github.com/qonto/prometheus-rds-exporter/internal/app/cloudwatch	87%	✔
github.com/qonto/prometheus-rds-exporter/internal/app/ec2	87%	✔
github.com/qonto/prometheus-rds-exporter/internal/app/exporter	81%	✔
github.com/qonto/prometheus-rds-exporter/internal/app/rds	67%	➖
github.com/qonto/prometheus-rds-exporter/internal/app/servicequotas	67%	➖
github.com/qonto/prometheus-rds-exporter/internal/app/trace	100%	✔
github.com/qonto/prometheus-rds-exporter/internal/app/unit	100%	✔
Summary	78% (1084 / 1383)	➖

Minimum allowed line rate is 60%

[dcupif]

LGTM 🚀