perf inject: Clarify build-id options a little bit

Update the documentation of --build-id and --buildid-all options to
clarify the difference between them.  The former requires full sample
processing to find which DSOs are actually used.  While the latter simply
injects every DSO's build-id from MMAP{,2} records, skipping SAMPLEs.
Reviewed-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: https://lore.kernel.org/r/20220923173142.805896-3-namhyung@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-inject.txt

@@ -25,10 +25,12 @@ OPTIONS
 -------
 -b::
 --build-ids::
-        Inject build-ids into the output stream
+	Inject build-ids of DSOs hit by samples into the output stream.
+	This means it needs to process all SAMPLE records to find the DSOs.
 
 --buildid-all::
-	Inject build-ids of all DSOs into the output stream
+	Inject build-ids of all DSOs into the output stream regardless of hits
+	and skip SAMPLE processing.
 
 --known-build-ids=::
 	Override build-ids to inject using these comma-separated pairs of