bitstream_filters: Correct dump_extradata description

The default is to dump extradata to keyframes, not all frames.
Also improve the description of the relevant AVOption.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

doc/bitstream_filters.texi

@@ -120,7 +120,7 @@ add extradata to all packets
 @end table
 @end table
 
-If not specified it is assumed @samp{e}.
+If not specified it is assumed @samp{k}.
 
 For example the following @command{ffmpeg} command forces a global
 header (thus disabling individual packet headers) in the H.264 packets

libavcodec/dump_extradata_bsf.c

@@ -81,7 +81,7 @@ fail:
 #define OFFSET(x) offsetof(DumpExtradataContext, x)
 #define FLAGS (AV_OPT_FLAG_VIDEO_PARAM|AV_OPT_FLAG_BSF_PARAM)
 static const AVOption options[] = {
-    { "freq", "When do dump extradata", OFFSET(freq), AV_OPT_TYPE_INT,
+    { "freq", "When to dump extradata", OFFSET(freq), AV_OPT_TYPE_INT,
         { .i64 = DUMP_FREQ_KEYFRAME }, DUMP_FREQ_KEYFRAME, DUMP_FREQ_ALL, FLAGS, "freq" },
         { "k",        NULL, 0, AV_OPT_TYPE_CONST, { .i64 = DUMP_FREQ_KEYFRAME }, .flags = FLAGS, .unit = "freq" },
         { "keyframe", NULL, 0, AV_OPT_TYPE_CONST, { .i64 = DUMP_FREQ_KEYFRAME }, .flags = FLAGS, .unit = "freq" },