[Format] Better document IPC file and stream equivalence

[pitrou]

Describe the enhancement requested

The IPC file and stream formats are roughly equivalent, but with some deviations. We should better document those.

See ML discussion at https://lists.apache.org/thread/jpxl3yzm96wkxzb1clokxklsy32b3plh

Component(s)

Format

[pitrou]

Issue resolved by pull request 49947
#49947