dtrace_callout_execute.4: Document the DTrace callout_execute provider

MFC after: 2 weeks
Fixes: 91dd9aae1ab8 Add explicit static DTrace tracing to the callout mechanism
Differential Revision: https://reviews.freebsd.org/D51397

(cherry picked from commit 3ccb2d9513e6a2e046e635c186da68acf8f8498b)