doc: add FFREPORT example

And fix a few nits.
Signed-off-by: Lou Logan <lou@lrcd.com>

doc/fftools-common-opts.texi

@@ -204,21 +204,29 @@ directory.
 This file can be useful for bug reports.
 It also implies @code{-loglevel verbose}.
 
-Setting the environment variable @code{FFREPORT} to any value has the
+Setting the environment variable @env{FFREPORT} to any value has the
 same effect. If the value is a ':'-separated key=value sequence, these
-options will affect the report; options values must be escaped if they
+options will affect the report; option values must be escaped if they
 contain special characters or the options delimiter ':' (see the
-``Quoting and escaping'' section in the ffmpeg-utils manual). The
-following option is recognized:
+``Quoting and escaping'' section in the ffmpeg-utils manual).
+
+The following options are recognized:
 @table @option
 @item file
 set the file name to use for the report; @code{%p} is expanded to the name
 of the program, @code{%t} is expanded to a timestamp, @code{%%} is expanded
 to a plain @code{%}
 @item level
-set the log level
+set the log verbosity level using a numerical value (see @code{-loglevel}).
 @end table
 
+For example, to output a report to a file named @file{ffreport.log}
+using a log level of @code{32} (alias for log level @code{info}):
+
+@example
+FFREPORT=file=ffreport.log:level=32 ffmpeg -i input output
+@end example
+
 Errors in parsing the environment variable are not fatal, and will not
 appear in the report.