Explain stream specifier usage for codec and other global options

[Ulf Zibis]

Summary of the bug:
At ​https://www.ffmpeg.org/ffmpeg-codecs.html#Codec-Options there is no hint that options should be postfixed by a stream specifier in case of ambiguity. An example would be helpful too.
At ​https://www.ffmpeg.org/ffmpeg.html#AVOptions the given example doesn't reflect the statement: "... a stream specifier should be attached ...", so "... in case of ambiguity" should be amended.
For https://ffmpeg.org/ffmpeg-formats.html#Format-Options the same applies.
How to reproduce:

% ffmpeg -i input -level:a 5 output # avoid ambiguity with video codec level
ffmpeg version: current

[Gyan]

At ​​https://www.ffmpeg.org/ffmpeg.html#AVOptions the given example doesn't reflect the statement

That example applies a format option. I'll add a new example for codec options.

[Gyan]

Example added in a9452fe6dc2ecf5b2249aae6d770655095b4e232