pvsystem.sapm documentation: clarification of coefficients used

[RDaxini]

Is your feature request related to a problem? Please describe.
pvlib.pvsystem.sapm requires effective irradiance and SAPM coefficients as inputs.

pvlib-python/pvlib/pvsystem.py

Lines 2169 to 2171 in 22364d8

	effective_irradiance : numeric
	Irradiance reaching the module's cells, after reflections and
	adjustment for spectrum. [W/m2]

pvlib-python/pvlib/pvsystem.py

Lines 2176 to 2178 in 22364d8

	module : dict-like
	A dict or Series defining the SAPM parameters. See the notes section
	for more details.

effective_irradiance considers AOI/spectrum. The notes section explaining module contains a table explaining the SAPM coefficients:

pvlib-python/pvlib/pvsystem.py

Lines 2194 to 2235 in 22364d8

	Notes
	-----
	The SAPM parameters which are required in ``module`` are
	listed in the following table.
	The Sandia module database contains parameter values for a limited set
	of modules. The CEC module database does not contain these parameters.
	Both databases can be accessed using :py:func:`retrieve_sam`.
	================ ========================================================
	Key Description
	================ ========================================================
	A0-A4 The airmass coefficients used in calculating
	effective irradiance
	B0-B5 The angle of incidence coefficients used in calculating
	effective irradiance
	C0-C7 The empirically determined coefficients relating
	Imp, Vmp, Ix, and Ixx to effective irradiance
	Isco Short circuit current at reference condition (amps)
	Impo Maximum power current at reference condition (amps)
	Voco Open circuit voltage at reference condition (amps)
	Vmpo Maximum power voltage at reference condition (amps)
	Aisc Short circuit current temperature coefficient at
	reference condition (1/C)
	Aimp Maximum power current temperature coefficient at
	reference condition (1/C)
	Bvoco Open circuit voltage temperature coefficient at
	reference condition (V/C)
	Mbvoc Coefficient providing the irradiance dependence for the
	BetaVoc temperature coefficient at reference irradiance
	(V/C)
	Bvmpo Maximum power voltage temperature coefficient at
	reference condition
	Mbvmp Coefficient providing the irradiance dependence for the
	BetaVmp temperature coefficient at reference irradiance
	(V/C)
	N Empirically determined "diode factor" (dimensionless)
	Cells_in_Series Number of cells in series in a module's cell string(s)
	IXO Ix at reference conditions
	IXXO Ixx at reference conditions
	FD Fraction of diffuse irradiance used by module
	================ ========================================================

I'm wondering whether any clarification over which coefficients are actually used is necessary. I think it's helpful to present the full table, but could someone be misled into thinking that since A0-A4 are required, a spectral correction is being applied (for example)

Describe the solution you'd like
Suggestion: add a line or two to the notes section clarifying which coefficients are used.

Describe alternatives you've considered
Such a revision could be redundant since effective_irradiance is already clearly (and correctly) defined. However, when I first glanced over these docs, I just felt like some clarification could help some users. Not a major issue; I am interested to hear whether anyone else thinks this would help or not.

[cwhanse]

I agree, that docstring should be clarified. The table is documenting what can be pulled from the (old) parameter file, rather than what is used by this function.