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

share/man/man9/counter.9

@@ -25,7 +25,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd March 6, 2020
+.Dd March 11, 2021
 .Dt COUNTER 9
 .Os
 .Sh NAME
@@ -98,10 +98,13 @@ or
 .Va M_WAITOK .
 If
 .Va M_NOWAIT
-is specified the operation may fail.
+is specified the operation may fail and return
+.Dv NULL .
 .It Fn counter_u64_free c
 Free the previously allocated counter
 .Fa c .
+It is safe to pass
+.Dv NULL .
 .It Fn counter_u64_add c v
 Add
 .Fa v

share/man/man9/zone.9

@@ -25,7 +25,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd August 20, 2020
+.Dd March 11, 2021
 .Dt UMA 9
 .Os
 .Sh NAME
@@ -385,6 +385,22 @@ specify an argument for the
 and
 .Dv dtor
 functions of the zone, respectively.
+The variants
+.Fn uma_zalloc_pcpu
+and
+.Fn uma_zfree_pcpu
+allocate and free
+.Va mp_ncpu
+shadow copies as described for
+.Dv UMA_ZONE_PCPU .
+If
+.Fa item
+is
+.Dv NULL ,
+then
+.Fn uma_zfree_pcpu
+does nothing.
+.Pp
 The
 .Fn uma_zalloc_domain
 function allows callers to specify a fixed