Document non-Vale/markdownlint tests in lint-doc.sh

What does this MR do?

As part of researching long options vs short options for Git in Verbose git command line arguments in empty rep... (!122213 - merged), I realized we had no style guide statement for our Git preferences. I knew, though, that we had a test to disallow short options in curl commands, though. I went looking in the style guide to find corroboration of the rule, and none existed.

Step 1 would be to put something into our testing page (https://docs.gitlab.com/ee/development/documentation/testing.html) that acknowledges lint-doc.sh does more than the markdownlint testing described in https://docs.gitlab.com/ee/development/documentation/testing.html#lint-checks. I've created a very bare-bones section - it may or may not be in the right place on the page - so we at least acknowledge we're doing these checks.

It should probably also be in the style guide, but I had a total of ten minutes to make this MVC, and this is as far as I got.

I have thoughts, but not a decision, on the original question (long/short options for Git?) but it will help to at least have the decision for curl documented as it's very similar.

Related issues

• Related to Verbose git command line arguments in empty rep... (!122213 - merged)
• Related to Use verbose command line flags in empty reposit... (#412650)

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