Clarify behavior of norm clipping

[timhoffm]

PR summary

Followup to #26449 (comment)

Closes #26446

Major correction: Clipping does not affect masked values

This can be seen in the following code, where the mask is reapplied

matplotlib/lib/matplotlib/colors.py

Lines 1936 to 1939 in f800bf6

	if clip:
	mask = np.ma.getmask(result)
	result = np.ma.array(np.clip(result.filled(vmax), vmin, vmax),
	mask=mask)

This code has been unmodified since 2016 and has been the same even before. I therefore believe it's better to just adapt the documentation to the existing behavior.

Other changes

Better explanation of the returned values for "outside" inputs and their interplay with colormaps.