perf kmem: Improve man page for record options

Since:

  https://lore.kernel.org/lkml/20200708183919.4141023-1-irogers@google.com/

The output option works for 'perf kmem', however, it must appear after
'record'. This is different to 'stat' where '-i' for the input must
appear before. Try to capture this complication in the man page.
Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lore.kernel.org/lkml/20210922212031.485950-1-irogers@google.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-kmem.txt

@@ -8,22 +8,25 @@ perf-kmem - Tool to trace/measure kernel memory properties
 SYNOPSIS
 --------
 [verse]
-'perf kmem' {record|stat} [<options>]
+'perf kmem' [<options>] {record|stat}
 
 DESCRIPTION
 -----------
 There are two variants of perf kmem:
 
-  'perf kmem record <command>' to record the kmem events
-  of an arbitrary workload.
+  'perf kmem [<options>] record [<perf-record-options>] <command>' to
+  record the kmem events of an arbitrary workload. Additional 'perf
+  record' options may be specified after record, such as '-o' to
+  change the output file name.
 
-  'perf kmem stat' to report kernel memory statistics.
+  'perf kmem [<options>] stat' to report kernel memory statistics.
 
 OPTIONS
 -------
 -i <file>::
 --input=<file>::
-	Select the input file (default: perf.data unless stdin is a fifo)
+	For stat, select the input file (default: perf.data unless stdin is a
+	fifo)
 
 -f::
 --force::