doc: Better documentation for the bitexact flag

Reviewed-by: Benoit Fouet <benoit.fouet@free.fr>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

doc/codecs.texi

@@ -71,7 +71,9 @@ Force low delay.
 @item global_header
 Place global headers in extradata instead of every keyframe.
 @item bitexact
-Use only bitexact stuff (except (I)DCT).
+Only write platform-, build- and time-independent data. (except (I)DCT).
+This ensures that file and data checksums are reproducable and match between
+platforms. Its primary use is for regression testing.
 @item aic
 Apply H263 advanced intra coding / mpeg4 ac prediction.
 @item cbp

doc/formats.texi

@@ -56,7 +56,9 @@ Enable RTP MP4A-LATM payload.
 @item nobuffer
 Reduce the latency introduced by optional buffering
 @item bitexact
-Only write binary portable, build and time independant data.
+Only write platform-, build- and time-independent data.
+This ensures that file and data checksums are reproducable and match between
+platforms. Its primary use is for regression testing.
 @end table
 
 @item seek2any @var{integer} (@emph{input})