Add getTextWidth etc. to docs

[imjosh]

Many useful methods, and in many cases necessary ones, are not included in the docs.

[Uzlopak]

isnt getTextWidth an internal method?

[imjosh]

Yes, I figured that out that now. Should getTextDimensions be used instead of getTextWidth? It's in the cell module but it seems like it should just be part of the main library. Also the documentation for it is incomplete. I'm just trying to center some text and all the solutions online say to use getTextWidth or they provide some partial, homebrew implementation of it with some of the features of getTextDimensions.

[HackbrettXXX]

Yes, getTextDimensions is the one you should use. And yes, the documentation is bad at many places. We are happy about every pull request that improves it.