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 <[e-mail]>
Signed-off-by: Namhyung Kim <[e-mail]>
Cc: Ian Rogers <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Stephane Eranian <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/Documentation/perf-inject.txt

···
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 -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