avformat/udp: clarify option description for timeout unit

Signed-off-by: Limin Wang <lance.lmwang@gmail.com>

libavformat/udp.c

@@ -138,7 +138,7 @@ static const AVOption options[] = {
     { "connect",        "set if connect() should be called on socket",     OFFSET(is_connected),   AV_OPT_TYPE_BOOL,   { .i64 =  0 },     0, 1,       .flags = D|E },
     { "fifo_size",      "set the UDP receiving circular buffer size, expressed as a number of packets with size of 188 bytes", OFFSET(circular_buffer_size), AV_OPT_TYPE_INT, {.i64 = 7*4096}, 0, INT_MAX, D },
     { "overrun_nonfatal", "survive in case of UDP receiving circular buffer overrun", OFFSET(overrun_nonfatal), AV_OPT_TYPE_BOOL, {.i64 = 0}, 0, 1,    D },
-    { "timeout",        "set raise error timeout (only in read mode)",     OFFSET(timeout),        AV_OPT_TYPE_INT,    { .i64 = 0 },      0, INT_MAX, D },
+    { "timeout",        "set raise error timeout, in microseconds (only in read mode)",OFFSET(timeout),         AV_OPT_TYPE_INT,  {.i64 = 0}, 0, INT_MAX, D },
     { "sources",        "Source list",                                     OFFSET(sources),        AV_OPT_TYPE_STRING, { .str = NULL },               .flags = D|E },
     { "block",          "Block list",                                      OFFSET(block),          AV_OPT_TYPE_STRING, { .str = NULL },               .flags = D|E },
     { NULL }