[PATCH v3 2/2] gdb/testsuite: Clarify -lbl option in gdb_test_multiple

[Thiago Jung Bauermann <thiago.bauermann@linaro.org>]

I was a bit confused about the -lbl option in gdb_test_multiple, and needed
to read its implementation to determine that it would be useful for my
needs.  Explicitly mention what the option does and why it's useful to
hopefully help other developers.
---
 gdb/testsuite/lib/gdb.exp | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/gdb/testsuite/lib/gdb.exp b/gdb/testsuite/lib/gdb.exp
index fe4ac7d27190..9a2a64fa9ba1 100644
--- a/gdb/testsuite/lib/gdb.exp
+++ b/gdb/testsuite/lib/gdb.exp
@@ -921,7 +921,10 @@ proc fill_in_default_prompt {prompt_regexp with_anchor} {
 #   if one of them matches.  If MESSAGE is empty COMMAND will be used.
 # -prompt PROMPT_REGEXP specifies a regexp matching the expected prompt
 #   after the command output.  If empty, defaults to "$gdb_prompt $".
-# -lbl specifies that line-by-line matching will be used.
+# -lbl specifies that line-by-line matching will be used.  This means
+#   that lines from GDB not matched by any pattern will be consumed from
+#   the output buffer.  This helps avoid buffer overflows and timeouts
+#   when testing verbose commands.
 # EXPECT_ARGUMENTS will be fed to expect in addition to the standard
 #   patterns.  Pattern elements will be evaluated in the caller's
 #   context; action elements will be executed in the caller's context.