Tweak y-axis label

[cbrnr]

Use more common y-axis label for PSD plot. Fixes #13296.

[cbrnr]

@drammock I tried to make the docstring slightly more readable, but the dB_plot_psd entry is just for the legacy function(s). The new functions are based on the _dB entry, which currently is

_dB = """
dB : bool
    Whether to plot on a decibel scale. If ``True``, plots
    10 × log₁₀({quantity}){caveat}.{extra}
"""

I would like to change the following three things:

1. The factor 10 only applies for PSDs, but for ASDs the factor is 20 (hopefully that's also what the code does, I didn't check).
2. The formula shows the conversion to dB, but the plots (both PSD and ASD) show dB/Hz or dB/√Hz. I think this should be included in the docs, but I'm not sure how.
3. In that docstring we could also include the reference power or amplitude values, but I'm also not sure how.