Update "Caching" section in User Guide

[var1abl3]

Environment

• pip version: latest
• Python version: all
• OS: all

Description
The "caching" section in the documentation is no longer accurate (and misleading). It states:
"When storing items in the cache, pip will respect the CacheControl header if it exists, or it will fall back to the Expires header if that exists. This allows pip to function as a browser would, and allows the index server to communicate to pip how long it is reasonable to cache any particular item."
However, max-age is hardcoded controller.py, and any such headers from the server are ignored:

149     resp = session.get(
150         url,
151         headers={
152             "Accept": "text/html",
153             # We don't want to blindly returned cached data for
154             # /simple/, because authors generally expecting that
155             # twine upload && pip install will function, but if
156             # they've done a pip install in the last ~10 minutes
157             # it won't. Thus by setting this to zero we will not
158             # blindly use any cached data, however the benefit of
159             # using max-age=0 instead of no-cache, is that we will
160             # still support conditional requests, so we will still
161             # minimize traffic sent in cases where the page hasn't
162             # changed at all, we will just always incur the round
163             # trip for the conditional GET now instead of only
164             # once per 10 minutes.
165             # For more information, please see pypa/pip#5670.
166             "Cache-Control": "max-age=0",
167         },
168     )

Expected behavior
The documentation should state the actual behavior.

How to Reproduce
N/A

Output
N/A

[gutsytechster]

Can I work on the issue? What I can understand is that we need to mention that pip uses its own Cache-Control settings in place of what is currently written.

[eamanu]

@gutsytechster are you still interest on working on this issue? If not, I can work on it. :)

[gutsytechster]

@eamanu you can go ahead with the issue. :)

[eamanu]

@gutsytechster thanks