plots: update docs

[pared]

after iterative/dvc#4602 we accept any target for dvc plots. Reflect that in docs.

• UPDATE: pending one thing (from plots and metrics: update new targets rules #1809 (comment)):

if we explain in the plots concept itself (plots ref. index) that plots are not only dvc.yaml based, people won't expect dvc.yaml to be present in the first place.
It's better to specify at the top level that plots files by definition could be anywhere.