ffmpeg -vstats_file content description

[Anan5a]

Summary of the bug:
I'm using FFmpeg to create a web encoding client. But the problem is there's no docs about the data produced by -vstats_file option.
Example:

out=  0 st=  0 frame=   718 q= 35.0 f_size=    894 s_size=     2877kB time= 29.793 br=   171.6kbits/s avg_br=   791.1kbits/s type= B out=  0 st=  0 frame=   719 q= -1.0 f_size=    824 s_size=     2878kB time= 29.834 br=   158.2kbits/s avg_br=   790.2kbits/s type= B
out=  0 st=  0 frame=   720 q= -1.0 f_size=    980 s_size=     2879kB time= 29.876 br=   188.2kbits/s avg_br=   789.4kbits/s type= B

In documentation there's no mention about which key means what. For example what does out= mean? st= ? also what are type=B/I/P ?
Please explain.
Also how am I supposed to know that the encoding process is completed?
How to reproduce:

$ ffmpeg -i input ... -vstats_file file.txt output
ffmpeg version
 4+
built on Debian 10

[Gyan]

Will add those details to the docs once my ffmpeg dev box is back.

[Stefano Sabatini]

Should be addressed in ​261fb55e.