Extend/clarify documentation related to filtergraph escaping

[idest]

1. In "doc/ffmpeg-all.html", chapter 39.61.2, "Text expansion", there's a phrase: "using a text file avoids these problems". Perhaps a cross-reference to -filter_script should be given after the phrase to specify the way of using a text file to load filter options.

2. Also, in the same file, chapter 32.2, "Notes on filtergraph escaping", cross-references to -filter_script and -filter_complex_script would perhaps be useful to specify ways of not having to handle multi-level escaping.

[idest]

Not sure which FFmpeg version to specify. ffmpeg -version reports 4.3.1, but I can't see it in the ticket's "Version" drop-down menu.

[Stefano Sabatini]

Should be fixed in ​7792f53e5e66eb3a340e93af12ab, thanks for the report.