Bring the dumpon(8) man page closer to reality:

• dumpon utility has not used kern.dumpdev sysctl since rev. 1.14 (sbin/dumpon/dumpon.c) when phk@ updated it to use the DIOCSKERNELDUMP ioctl [1]
• remove obsolete reference to sysctl(3)

While I am there, fix two style nits:

• use .Nm instead of `dumpon'
• change NOTES to IMPLEMENTATION NOTES, to bring it in line with recommended section headings in mdoc(7)

Original patch by: Martin Faxer <gmh003532brfmasthugget.se> [1]

PR: docs/39293
Approved and Reviewed by: des (mentor), re (scottl, bmah)