no documentation for string_agg

### What happens?

no documentation for string_agg, but it does exist!

https://github.com/malloydata/malloy/blob/5b9feda9a14db247b5a7cf372192ffbda3217f0c/test/src/databases/all/functions.spec.ts#L1371

### To Reproduce

search in docs. don't find it.

### OS:

macos

### Malloy Client:

local VSCode

### Malloy Client Version:

latest

### Database Connection:

postgres