hwpmc.4: correct stale default values and update diagnostics

The default values documented for kern.hwpmc.logbuffersize (4KB) and
kern.hwpmc.nbuffers_pcpu (64) have been incorrect since 2981a3420cb1
(2018), which updated the compiled defaults but did not update the
man page.

• Correct logbuffersize default from 4KB to 256KB, add 16MB maximum
• Correct nbuffers_pcpu default from 64 to 32, document 32MB per-CPU product limit with kern.hwpmc.logbuffersize
• Update DIAGNOSTICS section to reflect current warning messages

Reviewed by: mhorne
MFC after: 1 week
Sponsored by: NLINK (nlink.com.br)
Differential Revision: https://reviews.freebsd.org/D56050