Requirements file format documentation is incomplete

[maxwell-k]

Environment

• pip version: master
• Python version: all (Python 3.7)
• OS: all (Linux)

This is an issue with the documentation.

Description

The current requirements file format section of the documentation is incomplete and therefore misleading: https://pip.pypa.io/en/latest/reference/pip_install/#requirements-file-format

It can lead a reader to understand that certain content is not supported, when it is in fact supported.

Expected behavior

The requirements file format documentation should be a comprehensive explanation of what is permitted in a requirements file. A reader should be able to identify what is allowed from the documentation without having to read the underlying source code.

How to Reproduce

1. Browse to https://pip.pypa.io/en/latest/reference/pip_install/#requirements-file-format
2. Compare the list under "The following options are supported:" with
3. SUPPORTED_OPTIONS in https://github.com/pypa/pip/blob/master/src/pip/_internal/req/req_file.py#L60

Output

Five supported options are missing from the list:

• -c, --constraint
• -e, --editable
• -r, --requirement
• --pre
• -Z, --always-unzip