docstring for UnitBox

[Mattriks]

• I've built the docs and confirmed these changes don't cause new errors

This PR

• adds docstrings for UnitBox (mentioned in line does not support relative measures #335)
• Note there is already an example of UnitBox in the Tutorial

[codecov-io]

Codecov Report

Merging #377 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #377   +/-   ##
=======================================
  Coverage   40.81%   40.81%           
=======================================
  Files          18       18           
  Lines        3058     3058           
=======================================
  Hits         1248     1248           
  Misses       1810     1810

Impacted Files	Coverage Δ
src/container.jl	38.28% <ø> (ø)	⬆️
src/measure.jl	45.45% <ø> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 95e3458...41b58d6. Read the comment docs.