[FFmpeg-user] Improved documentation of -filter_complex_script

[Gordon Charles]

The documentation for filter_complex_script reads:

-filter_complex_script filename (global)

This option is similar to -filter_complex, the only difference is that 
its argument is the name of the file from which
a complex filtergraph description is to be read.

Unlike the filter_complex command line option which supports multi-line 
input through the shell supported methods,
I could not find:

1) an example of a multi-line input
2) a statement that multi-line input is not supported

I was constructing a python script which would use a text file with 
start and stop times to trim sections from an av
file while maintaining the time stamps.  debugging the filter generation 
in a multi-line format would have been
easier, so I was looking for a solution but could not find one.  I 
eventually settled on a multi-line I could issue
from the command line and a programmatic switch to change the content to 
a single line for filter_complex_script.

I believe it would be useful to include the limitation if the script 
must be a single line in the documentation or
provide documentation for multi-line if it is supported.

Regards,

Gordon