Documents the scanner.speed config and minio_scanner_speed envvar (#973)

Closes #796

And a few other paragraphs here there and yon.

source/includes/common/scanner.rst

@@ -0,0 +1,43 @@
+.. start-scanner-speed-config
+
+You can adjust how MinIO balances the scanner performance with read/write operations using either the :envvar:`MINIO_SCANNER_SPEED` environment variable or the :mc-conf:`scanner speed <scanner.speed>` configuration setting.
+
+.. end-scanner-speed-config
+
+
+.. start-scanner-speed-values
+
+MinIO utilizes the scanner for :ref:`bucket replication <minio-bucket-replication>`, :ref:`site replication <minio-site-replication-overview>`, and :ref:`lifecycle management <minio-lifecycle-management>` tasks.
+
+Valid values include:
+
+.. list-table::
+   :stub-columns: 1
+   :widths: 30 70
+   :width: 100%
+   
+   * - ``fastest``
+     - Removes scanner wait on read/write latency, allowing the scanner to operate at full-speed and IOPS consumption.
+       This setting may result in reduced read and write performance.
+   
+   * - ``fast``
+     - Sets a short scanner wait time on read/write latency, allowing the scanner to operate at a higher speed and IOPS consumption.
+       This setting may result in reduced read and write performance.
+   
+   * - ``default``
+     - Sets a moderate scanner wait time on read/write latency, allowing the scanner to operate at a balanced speed and IOPS consumption.
+       This setting seeks to maintain read and write performance while allowing ongoing scanner activity. 
+   
+   * - ``slow``
+     - Sets a medium scanner wait time on read/write latency, where the scanner operates at a reduced speed and IOPS consumption.
+       This setting allows better read and write performance while reducing scanner performance.
+
+       May impact scanner-dependent features, such as lifecycle management and replication.
+
+   * - ``slowest``
+     - Sets a large scanner wait time on read/write latency, where the scanner operates at a substantially lower speed and IOPS consumption.
+       This setting prioritizes read and write operations at the potential cost of scanner operations.
+
+       May impact scanner-dependent features, such as lifecycle management and replication.
+
+.. end-scanner-speed-values