TL;DR: the Method enum only supports fraction of all the registered HTTP methods.

EDIT: I've currently opened the PR #332 to resolve this issue.

About Mozilla's documentation

Mozilla's HTTP request methods only contains information about the methods specified in RFC 7231 and RFC 5789. While these are the most commonly used ones, they only account for 9 out of the 39 (~23%) currently registered HTTP methods.

What is a Registered HTTP Method?

RFC 7231 states the following:

4 Request Methods
4.1 Overview

...

Additional methods, outside the scope of this specification, have
been standardized for use in HTTP. All such methods ought to be
registered within the "Hypertext Transfer Protocol (HTTP) Method
Registry" maintained by IANA, as defined in Section 8.1.

...

8 IANA Considerations
8.1 Method Registry
The "Hypertext Transfer Protocol (HTTP) Method Registry" defines the
namespace for the request method token (Section 4). The method
registry has been created and is now maintained at
http://www.iana.org/assignments/http-methods.

Essentially a registered HTTP method is a method that has gone through the process of being added to IANA's HTTP Method Registry.