Update plotting docstrings

[leburgel]

Updates the plotting docstrings to state that users need to add using Plots themselves before being able to produce any plots.

See #246.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ
src/utility/plotting.jl	86.36% <100.00%> (ø)

... and 2 files with indirect coverage changes

[lkdvos]

Would it be possible to make a slightly more clear separation between arguments and keyword arguments? With them listed like this, you need to have noticed that there is a semicolon in the signature, which seems a bit easy to miss

[leburgel]

Would it be possible to make a slightly more clear separation between arguments and keyword arguments?

Done, and also updated the signature itself (the transferplot one didn't even have the semicolon...).