Rename our "GitLab Flavored Markdown" acronym from "GFM" to "GLFM"

Some confusion has been raised concerning our overloading of the acronym "GFM". Most of the world knows it as "GitHub Flavored Markdown", which refers to a specific set of extensions to Markdown/CommonMark used on GitHub. When another product is referred to as being "GFM" compatible, they are referring to these specific extensions.

We overload the meaning of "GFM" to mean "GitLab Flavored Markdown", which is a superset of GitHub's version. However it can cause confusion as they are not the same thing.

• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22122#note_106634498
• https://talk.commonmark.org/t/commonmark-now-in-use-on-gitlab/2944/2
• mentioned in another issue I can't locate right now

We should discuss using a different acronym.