neubot.1.rst: better dash rate-adaptation descr

What matters is not the two seconds threshold, which is an
implementation detail, but that we request the next segment
using the rate representation that is close to the speed
with which we downloaded the previous chunk.

(There is also the case in which we overcome the two seconds
threshold, which is not documented as of now. However, the
"in the common case" is there exactly to hint that what we say
is not exactly the full story. I will try to come up with an
even better description of dashtest in the future.)

doc/neubot.1.rst

@@ -119,12 +119,11 @@ the beginning of the test, the dashtest client requests the first segment
 using the lowest bitrate representation. During the download of the
 first segment, the client calculates the estimated available bandwidth
 of the downloaded segment by dividing the size of such segment (in kbit)
-by the download time (in seconds). If the download lasted less than two
-seconds, the client requests a bigger next segment; otherwise, if the
-download lasted more than two seconds, the client requests a smaller
-next segment. This process is, of course, repeated for all subsequent
-segments, thereby adapting the requested bitrate representation to
-the download speed.
+by the download time (in seconds). Next, the Dashtest requests
+the next segment using, in the common case, the representation rate
+that is closer to the download speed of the current segment. This process
+is, of course, repeated for all subsequent segments, thereby adapting
+the requested bitrate representation to the download speed.
 
 
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