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

Closes #796

And a few other paragraphs here there and yon.

source/administration/bucket-replication.rst

@@ -203,6 +203,10 @@ feature. The time required to fully synchronize a bucket depends on a number of
 factors, including but not limited to the current cluster replication load,
 overall cluster load, and the size of the namespace (all objects in the bucket).
 
+.. include:: /includes/common/scanner.rst
+   :start-after: start-scanner-speed-config
+   :end-before: end-scanner-speed-config
+
 If versioning was not previously enabled when configuring bucket replication, 
 existing objects have a ``versionid = null``. These objects do replicate.
 

source/administration/object-management/object-lifecycle-management.rst

@@ -123,6 +123,10 @@ Scanner performance typically depends on the available node resources, the size
 For example, a cluster that starts with 100TB of data that then grows to 200TB of data may require more time to scan the entire namespace of buckets and objects given the same hardware and workload.
 As the cluster or workload increases, scanner performance decreases as it yields more frequently to ensure priority of normal S3 operations.
 
+.. include:: /includes/common/scanner.rst
+   :start-after: start-scanner-speed-config
+   :end-before: end-scanner-speed-config
+
 Consider regularly checking cluster metrics, capacity, and resource usage to ensure the cluster hardware is scaling alongside cluster and workload growth:
 
 - :ref:`minio-metrics-and-alerts`