doc/formats: document f_strict/strict for AVFormatContext

document f_strict/strict option for AVFormatContext

Signed-off-by: Jun Zhao <barryjzhao@tencent.com>

doc/formats.texi

@@ -224,6 +224,28 @@ would require too many resources due to a large number of streams.
 @item skip_estimate_duration_from_pts @var{bool} (@emph{input})
 Skip estimation of input duration when calculated using PTS.
 At present, applicable for MPEG-PS and MPEG-TS.
+
+@item strict, f_strict @var{integer} (@emph{input/output})
+Specify how strictly to follow the standards. @code{f_strict} is deprecated and
+should be used only via the @command{ffmpeg} tool.
+
+Possible values:
+@table @samp
+@item very
+strictly conform to an older more strict version of the spec or reference software
+@item strict
+strictly conform to all the things in the spec no matter what consequences
+@item normal
+
+@item unofficial
+allow unofficial extensions
+@item experimental
+allow non standardized experimental things, experimental
+(unfinished/work in progress/not well tested) decoders and encoders.
+Note: experimental decoders can pose a security risk, do not use this for
+decoding untrusted input.
+@end table
+
 @end table
 
 @c man end FORMAT OPTIONS