Update docs to explain config option merging

[danny-andrews]

What is the purpose of this pull request? (put an "X" next to item)

[X] Documentation update
[ ] Bug fix (template)
[ ] New rule (template)
[ ] Changes an existing rule (template)
[ ] Add autofixing to a rule
[ ] Add a CLI option
[ ] Add something to the core
[ ] Other, please explain:

What changes did you make? (Give an overview)

Updated the docs to explain how config options are merged from base configs.

Is there anything you'd like reviewers to focus on?

There was a lot of confusion generated by the merging behavior of rules inherited from base configs, so I thought I would clear that up here. There still may be scenarios which arise where options cannot be overridden from base configs, but we'll deal with those on a per-case basis.

[eslintbot]

Thanks for the pull request, @danny-andrews! I took a look to make sure it's ready for merging and found some changes are needed:

• The commit summary needs to begin with a tag (such as Fix: or Update:). Please check out our guide for how to properly format your commit summary and update it on this pull request.

Can you please update the pull request to address these?

(More information can be found in our pull request guide.)

[mention-bot]

@danny-andrews, thanks for your PR! By analyzing the history of the files in this pull request, we identified @pedrottimark, @nzakas and @MikeMcl to be potential reviewers.

[not-an-aardvark]

Thanks for the PR! I think the build is failing because markdownlint wants you to use four spaces in markdown indentation, rather than two.

[jsf-clabot]

All committers have signed the CLA.

[eslintbot]

Thanks for the pull request, @danny-andrews! I took a look to make sure it's ready for merging and found some changes are needed:

• The commit summary needs to begin with a tag (such as Fix: or Update:). Please check out our guide for how to properly format your commit summary and update it on this pull request.

Can you please update the pull request to address these?

(More information can be found in our pull request guide.)

[nzakas]

Can you use the "error", "warning", "off" notation instead of the numeric notation for severity?

[danny-andrews]

Done.

[kaicataldo]

@danny-andrews Sorry to have you do this again, but mind signing the JS Foundation CLA by clicking on "Details" next to the license/cla check in the checks below?

[eslintbot]

Thanks for the pull request, @danny-andrews! I took a look to make sure it's ready for merging and found some changes are needed:

• The commit summary needs to begin with a tag (such as Fix: or Update:). Please check out our guide for how to properly format your commit summary and update it on this pull request.

Can you please update the pull request to address these?

(More information can be found in our pull request guide.)

[eslintbot]

Thanks for the pull request, @danny-andrews! I took a look to make sure it's ready for merging and found some changes are needed:

• The commit summary needs to begin with a tag (such as Fix: or Update:). Please check out our guide for how to properly format your commit summary and update it on this pull request.

Can you please update the pull request to address these?

(More information can be found in our pull request guide.)

[platinumazure]

LGTM, thanks for the contribution!

[danny-andrews]

If this looks ready to go, I'll squash and amend the commit message.

[nzakas]

LGTM

[nzakas]

@danny-andrews don't worry, we can squash and change the commit message directly. Thanks for the contribution!