docs: add clarifications to read timeout

[cojenco]

Clarify timeout definition and usage, especially on read timeout

• read timeout is the number of seconds the client will wait for the server to send a response.
  In most cases, this is the maximum wait time before the server sends the first byte.
• connect timeout is the number of seconds to establish a connection to the server

Fixes internal b/238779055

[andrewsg]

Is this information accurate for non-media API calls as well as media calls? I remember there being some nuance there even though both of them boil down to the requests library.