pmc_allocate(3): document the count argument

This was added in b2ca2e50b9, and serves to provide an initial value
to the PMC, eliminating the need for a second syscall via pmc_set(3).

Reviewed by:	gnn, 0mp (manpages)
MFC after:	1 week
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D29885

lib/libpmc/pmc_allocate.3

@@ -23,7 +23,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd September 22, 2008
+.Dd April 21, 2021
 .Dt PMC_ALLOCATE 3
 .Os
 .Sh NAME
@@ -41,6 +41,7 @@
 .Fa "uint32_t flags"
 .Fa "int cpu"
 .Fa "pmc_id_t *pmcid"
+.Fa "uint64_t count"
 .Fc
 .Ft int
 .Fn pmc_release "pmc_id_t pmc"
@@ -111,6 +112,20 @@ Process scope PMC allocations should specify the constant
 .Dv PMC_CPU_ANY
 for this argument.
 .Pp
+The
+.Fa count
+argument behaves identically to the
+.Xr pmc_set 3
+function's
+.Fa value
+argument.
+For counting PMCs,
+.Fa count
+specifies the initial value of the allocated PMC.
+For sampling PMCs,
+.Fa count
+specifies the reload count.
+.Pp
 Function
 .Fn pmc_release
 releases the PMC denoted by argument