pmc_allocate(3): document the count argument

[mhorne]

Summary

This was added in b2ca2e50b9aa, and serves to provide an initial value
to the PMC, eliminating the need for a second syscall via pmc_set(3).

[0mp]

OK from manpages. Please remember to bump Dd.

lib/libpmc/pmc_allocate.3

 .else
 X${COMPILER}?=  ${${COMPILER}}
 .endif
 .endfor
 # If a full path to an external cross compiler is given, don't build
 # a cross compiler.
 .if ${XCC:N${CCACHE_BIN}:M/*}
 MK_CLANG_BOOTSTRAP=     no
+# Make sure sub-makes see the option as disabled so the hack in bsd.sys.mk to
+# work around incompatible headers in Clang's resource directory is enabled.
+.MAKEOVERRIDES+=        MK_CLANG_BOOTSTRAP
 .endif
 
 # Pull in compiler metadata from buildworld/toolchain if possible to avoid
 # running CC from bsd.compiler.mk.
 .if make(installworld) || make(install) || make(distributeworld) || \
     make(stageworld)
 .-include "${OBJTOP}/toolchain-metadata.mk"
 .if !defined(_LOADED_TOOLCHAIN_METADATA)