[3.5] Document FormData and clarify possible types of ClientSession.request's data argument (#3755) (#3786)
[3.5] Document FormData and clarify possible types of ClientSession.request's data argument (#3755) (#3786)

(cherry picked from commit 9876e294f)

Co-authored-by: JustAnotherArchivist <e8f78b9b7ad274a5f3158d37ebac9b6d40db59b9@users.noreply.github.com>
KeepSafe/aiohttp,KeepSafe/aiohttp,KeepSafe/aiohttp