Lack of documentation for GIS in MariaDB KB

In 10.1.3 we are going to have new GIS features that need to be documented (see the separate email from Holyfoot); but I also found that our existing documentation for GIS is scarce and apparently incomplete. I started with the simple function ST_AsText(...), which is apparently a synonym for AsText(...), but couldn't find any mention of it. I suppose there are many more missing.

I also had a brief chat with a user who, after taking a quick look at our KB, said that "maria gis implementation is still a bit poor" – which might be true to some extent, but I suppose the lack of documentation is also a part of the problem which creates this impression.

The area needs to be revised and consistently documented.