Document `uparse`

Currently, searching for `uparse` in the docs (dev) gives one unrelated result:

Maybe an example of using `uparse` could be added to the docs?

Also related to the question I just asked in https://github.com/PainterQubits/Unitful.jl/issues/214, maybe add a function that does the reverse of `uparse`, i.e., that takes a `Quantity` and returns a valid (`uparse`-able) `String`?

Start by searching the documentation for `uparse` and review the related discussion in issue 214. Done should include a documented example of using `uparse`; clarify separately whether generating a valid `uparse`-able string from a `Quantity` is part of this issue.

Written by the indexing model from the issue text.