Document terminal coloring in the -loglevel option.

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@23633 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

doc/fftools-common-opts.texi

@@ -82,4 +82,8 @@ Set the logging level used by the library.
 @item debug
 @end table
 
+By default the program logs to stderr, if coloring is supported by the
+terminal, colors are used to mark errors and warnings. Log coloring
+can be disabled setting the environment variable @env{NO_COLOR}.
+
 @end table