lavc: extend documentation for the "bf" option

doc/codecs.texi

@@ -172,7 +172,13 @@ Set max video quantizer scale (VBR). Must be included between -1 and
 Set max difference between the quantizer scale (VBR).
 
 @item bf @var{integer} (@emph{encoding,video})
-Set max number of B frames.
+Set max number of B frames between non-B-frames.
+
+Must be an integer between -1 and 16. 0 means that B-frames are
+disabled. If a value of -1 is used, it will choose an automatic value
+depending on the encoder.
+
+Default value is 0.
 
 @item b_qfactor @var{float} (@emph{encoding,video})
 Set qp factor between P and B frames.

libavcodec/options_table.h

@@ -105,7 +105,7 @@ static const AVOption avcodec_options[] = {
 {"qmin", "minimum video quantizer scale (VBR)", OFFSET(qmin), AV_OPT_TYPE_INT, {.i64 = 2 }, -1, 69, V|E},
 {"qmax", "maximum video quantizer scale (VBR)", OFFSET(qmax), AV_OPT_TYPE_INT, {.i64 = 31 }, -1, 1024, V|E},
 {"qdiff", "maximum difference between the quantizer scales (VBR)", OFFSET(max_qdiff), AV_OPT_TYPE_INT, {.i64 = 3 }, INT_MIN, INT_MAX, V|E},
-{"bf", "use 'frames' B frames", OFFSET(max_b_frames), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, -1, FF_MAX_B_FRAMES, V|E},
+{"bf", "set maximum number of B frames between non-B-frames", OFFSET(max_b_frames), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, -1, FF_MAX_B_FRAMES, V|E},
 {"b_qfactor", "QP factor between P- and B-frames", OFFSET(b_quant_factor), AV_OPT_TYPE_FLOAT, {.dbl = 1.25 }, -FLT_MAX, FLT_MAX, V|E},
 {"rc_strategy", "ratecontrol method", OFFSET(rc_strategy), AV_OPT_TYPE_INT, {.i64 = DEFAULT }, INT_MIN, INT_MAX, V|E},
 {"b_strategy", "strategy to choose between I/P/B-frames", OFFSET(b_frame_strategy), AV_OPT_TYPE_INT, {.i64 = 0 }, INT_MIN, INT_MAX, V|E},