docs: Improve --user/--no-user documentation

[Ashutosh0x]

Summary

This PR improves the documentation for the --user and --no-user install options as requested in #12924.

Changes

Documentation Updates (docs/html/user_guide.rst)

1. Added 'Automatic User Install Fallback' section: Documents the often-surprising behavior where pip automatically falls back to a user install when site-packages is not writeable.

2. Added '--no-user Option' section: Documents the previously hidden --no-user option and its use cases.

3. Added 'Environment Variables' section: Documents PIP_USER for controlling user install behavior, and PYTHONUSERBASE for customizing install location.

Source Code Changes (src/pip/_internal/commands/install.py)

1. Made --no-user visible in help: Changed from SUPPRESS_HELP to proper help text explaining the option.

News Fragment

Added
ews/12924.doc.rst documenting the changes.

Fixes

Closes #12924

Testing

• Documentation builds correctly
• --no-user option now shows in pip install --help

[notatallshaw]

Why did this change? Seems like noise.

[notatallshaw]

Same here, why did this change?

[notatallshaw]

This news item is way too long, news items appear as bullet points in the change log. As a rule of thumb keep all news items to a short one or two sentences. For example this could just be:

Improved documentation for the ``--user`` and ``--no-user`` options.

[notatallshaw]

--no-user was intentionally hidden when it was created due to concerns of behavior with --user and --no-user together, see #5116 (comment) and #5085 (comment)

I don't think we can document or this feature without making them mutually exclusive, and I don't think we can do that without warning users first, we could need to trigger a deprecation warning for two cycles if --user and --no-user are called, and then make --no-user visible, mutually exclusive with --user, and document it.