Docs: Change rule descriptions for consistent casing

[btmills]

What issue does this pull request address?

Scrolling through http://eslint.org/docs/rules, I found two descriptions that began with uppercase letters, while all the rest were lowercase.

What changes did you make? (Give an overview)

I changed the two inconsistent descriptions to begin with lowercase letters.

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

For the curious, here's how I ran a quick check to be sure I didn't miss any:

Array.from(document.querySelectorAll('tr td:last-child p'))
    .map(p => p.innerText)
    .filter(description =>
        description[0].toLowerCase() !== description[0]
    )

[mention-bot]

@btmills, thanks for your PR! By analyzing the annotation information on this pull request, we identified @gyandeeps, @jfmengels and @kaicataldo to be potential reviewers

[eslintbot]

LGTM

[ilyavolodin]

LGTM. We should think about adding a custom rule for this.

[vitorbal]

@ilyavolodin sounds good, maybe the custom rule could also enforce that the description starts with one of "enforce", "require", "disallow", etc.?
When we make it so the title of each rule page also comes from the meta.description we will benefit even more from this consistency.