Documentation: security, JTAG/SWD locking

[lawrencek52]

The documentation in https://github.com/pyocd/pyOCD/blob/main/docs/security.md does a great job of describing how to unlock a chip if it is locked (although it seems a little out of date, some of the sample commands returned errors). But security.md fails to describe how to lock a chip if it is unlocked.