docs: clarify timeout parameter behavior

[Yanhu007]

Summary

Fixes #7350 — Clarify the timeout parameter docstring in requests.get() and related functions.

Changes

Expanded the :param timeout: docstring to explain:

1. Single float = both timeouts: When a single float is provided, it applies as both the connect timeout and the read timeout
2. Not a total limit: This is not a total time limit on the entire response — it applies per-recv() call
3. Tuple for fine control: A (connect, read) tuple can be used to set them independently

Before

How many seconds to wait for the server to send data before giving up, as a float, or a (connect timeout, read timeout) tuple.

After

How many seconds to wait for the server to send data before giving up, as a float, or a (connect timeout, read timeout) tuple. When a single float is provided, it is used as both the connect and the read timeout. This is not a total time limit on the entire response download; it applies separately to the connection phase and to each recv() call while waiting for data from the server.