Change the documentation for the requirements file format so that it
matches the implementation [0]. Also change the order of the options in
the implementation so that the documentation reads better.

Before this change the documentation included an incomplete list of
supported options.

This change adds the missing options and changes the order to match, so
that the two locations are easier to keep in sync. After this change the
list in the documentation matches SUPPORTED_OPTIONS in
src/pip/_internal/req/req_file.py

[0]: https://github.com/pypa/pip/blob/master/src/pip/_internal/req/req_file.py#L60

In the documentation the links for the pip commands index options
are mixed up.

The index options are common to multiple commands, but in the
documentation they should be specific to a command for the links to
point to the right chapter.

GitHub: #7347

Cache wheels that are built from Git requirements
that contain an immutable revision (i.e. a sha).

Fixes #7182

This section has caused a lot of user confusion since PEP 517/518. It seems that users are referring to this section for advice about how to deal with build time issues, which contains statements that do not hold true for build requirements.

This example did not make sense unless it was possible to install specify a
version of a package one installs referencing by the path (here: `.`).
A new example for the local installation with an extra has been added
instead.

fix #5888

Creating a news entry from the pull request as a separate commit.  Does this warrant an issue?

Python 3 docs are more actively maintained and are the future of the
project.

PEP 426 was withdrawn, so point to the relevant
accepted PEPs where appropriate.