Docs: Describe values under Extending Configuration Files (refs #6240)

[pedrottimark]

For this docs counterpart to #6248 let’s aim for about 3 steps of critique:

1. Baseline with only placeholder content for "eslint:all"
   • Critique a new structure for Extending Configuration Files that is adapted from new structure under Options in rule docs: bulleted list of extends property values followed by level-3 headings with an example of configuration.
   • Critique reorganization and rewriting of existing content. Is it correct, clear, and complete in context? For example, this section is the target of the link about "eslint:recommended" on the rules index page. In particular, can whoever has expert knowledge ( @IanVS ? :) help me with the paragraph about “ESLint resolves a relative path…”
2. Baseline improvements plus first draft of new content about "eslint:all" including thoughts from Configuration to enable all rules #6240 and New: Add eslint:all option (fixes #6240) #6248 especially New: Add eslint:all option (fixes #6240) #6248 (comment)
3. Second draft of new content about "eslint:all"

[eslintbot]

LGTM

[mention-bot]

By analyzing the blame information on this pull request, we identified @nzakas, @fakewaffle and @gyandeeps to be potential reviewers

[nzakas]

Why the quotes around these terms? If the intent is to introduce new terms, then I'd suggest formally defining them in a terminology section.

[eslintbot]

LGTM

[pedrottimark]

Made changes from first critique, asks self “what was I thinking?”

Added example sentences to make config file format explicit and changed one to YAML.

Next up: first draft of content and example for "eslint:all"

[eslintbot]

LGTM

[pedrottimark]

Bring all feedback great and small to first draft of "eslint:all"

[ilyavolodin]

Nice, LGTM. Should we put an even bigger disclaimer around eslint:all section. Using this will void your warranty and don't come crying to us?:-)

[nzakas]

Yeah, I might just put this directly under the heading:

Important: This configuration changes with every minor and major release, and as such, is not recommended for production use. Use at your own risk.

[eslintbot]

LGTM

[pedrottimark]

Included Important paragraph after initial descriptive paragraph because it seemed to stand out a bit more there and because the following paragraph explains why.

Changed the first and last level-3 headings to give more descriptive anchors:

• Using "eslint:recommended"
• Using "eslint:all"

s/specify different options/override default options/

Changed from bulleted items to ordinary paragraphs. Rephrased to include original use case: especially if you rarely override options or disable rules.

[nzakas]

LGTM

[pedrottimark]

The first two paragraphs might work as a short summary for the version blog with link from "eslint:all" to the subheading in this doc.

The extends property value can be "eslint:all" to enable all core rules in the currently installed version of ESLint. The set of core rules can change at any minor or major version of ESLint.

Important: This configuration is not recommended for production use because it changes with every minor and major version of ESLint. Use at your own risk.

[nzakas]

Yeah, I'll be sure to include that.