Update sql-support reference doc with added functions

[samjewell]

These functions were added in these PRs:

• implement stddev_pop(), stddev_samp(), var_pop(), var_samp(), and aliases go-mysql-server#2928
• implement ntile go-mysql-server#2935
• fix ntile syntax vitess#410

[samjewell]

I'm actually opening this PR because I was curious to know if you had any automation here at all, to automatically update this doc page in response to the actual support you have implemented. Doesn't seem to be the case as far as I can tell, reading the file itself and looking at the history of changes on the file, but please let me know. Thanks!

[samjewell]

ah damn. I saw in the readme that I should make the PR into gitbook-dev first, but when I updated the base-branch to that from gitbook-publish I see a LOT of changes. I guess I forked while I was looking at gitbook-publish branch on Github, and I must have landed there from the docs themselves, when I clicked Edit on GitHub.

Long shot, but do you fancy just cherry-picking my commit and merging it in? I wonder if that would be less work than me trying to rebase, across repos.

[jycor]

LGTM