Document that uma_zfree_pcpu() allows NULL now

While here also document that for counter_u64_free().

Reviewed by: rpokala@
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D29215

(cherry picked from commit 51dc8e7f688867e73eb7edc6bc65fdc77c9d5fff)