Help with documentation 2021 edition

I'm looking for people to help finish the documentation for the 2021 edition. If you are interested, the following issues need some help:

Each of them should contain some instructions on what needs to be done, though feel free to ask questions if it isn't clear. If you want to work on one, be sure to assign yourself first (@rustbot claim) so that multiple people don't work on the same ones. The reference also has a style guide for guidance on how to format things. The README contains instructions on how to run tests.