Docs: document Undertow system-property 'io.undertow.disable-file-system-watcher'

There has been added a new system-property in Undertow io.undertow.disable-file-system-watcher, see linked Jira. Property was added in EAP7.0.1 version. This should be documented with brief description in EAP7 Configuration Guide.

To be honest I am not sure about appropriate place. Maybe create some section System properties at the end of the Undertow chapter?

NOTE: please be aware that this system property is valid for EAP7.0.x versions only as it is overrided in EAP7.1 by new configuration attribute in Undertow subsystem /subsystem=undertow/servlet-container[disable-file-watch-service]. For more information see JBEAP-6027.