Make string manipulation functions easier to find

[danielcompton]

slice, and other functions work on many different types including strings, so they don't get put into the string manipulation section in the docs. I was looking for something like slice, but thought RethinkDB was missing it because it wasn't in the string manipulation section. I'm not sure what the answer is here. A few possibilities:

• Leave it the same, people will learn that many RethinkDB functions work on multiple types
• Create a way to view functions by the type they work on, rather than the category (sounds like a lot of work)
• Write a guide on string manipulation with RethinkDB which collates all of the string functions together.

I think a guide could be a good option. To me string manipulation feels like a fundamentally different operation from operating on tables, sequences, or arrays, showing what options you have would be helpful.

OTOH, I think it's only slice, count, coerceTo that operate on strings and don't sit in the string manipulation section.

[danielmewes]

There's also the less generic option of having these functions appear twice in the command list, once in their current place and once in the string manipulation section. I'm not sure how well that's going to work though.

add is another relevant command by the way.