scheduler: document localhost platform

Author: oliver-sanders

src/user-guide/writing-workflows/scheduler.rst

@@ -230,3 +230,20 @@ Example:
    .. cylc-scope::
 
 For more information see: :cylc:conf:`global.cylc[scheduler][run hosts]ranking`.
+
+Platform Configuration
+^^^^^^^^^^^^^^^^^^^^^^
+
+From the perspective of a running :term:`scheduler` ``localhost`` is the
+scheduler host.
+
+The ``localhost`` platform is configured by
+:cylc:conf:`global.cylc[platforms][localhost]`.
+
+It configures:
+
+* Jobs that run on the ``localhost`` platform. I.E. any jobs which have
+  :cylc:conf:`[runtime][<namespace>]platform=localhost` or which don't have a
+  platform configured.
+* Connections to the scheduler hosts (e.g. the
+  :cylc:conf:`ssh command <global.cylc[platforms][<platform name>]ssh command>`).