doc: param tables

Author: cadenmyers13

doc/source/snippets/data_params.rst

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+.. list-table::
+   :widths: 2 6 2 2
+   :header-rows: 1
+   :class: wrap-text
+
+   * - PDF Parameter
+     - Description
+     - Refineable?
+     - Structural or Instrumental?
+   * - ``scale``
+     - Overall scaling factor between calculated and observed PDFs; sets the absolute intensity.
+     - Yes
+     - Instrumental
+   * - ``qdamp``
+     - Damping of PDF peaks due to limited experimental Q-range. Typically affects high-r ripples and overall peak amplitudes.
+     - Yes
+     - Instrumental
+   * - ``qbroad``
+     - Instrumental or sample-related broadening of PDF peaks. This is due to the fact that diffraction peaks have different widths as a function of Q.
+     - Yes
+     - Instrumental
+   * - ``qmin``
+     - Minimum Q included in the fit; sets the lower bound of the S(Q) integration range.
+     - No, from experiment/setup
+     - Instrumental
+   * - ``qmax``
+     - Maximum Q included in the fit; sets the upper bound of the S(Q) integration range.
+     - No, from experiment/setup
+     - Instrumental
+   * - ``rmin``
+     - Minimum r (distance) included in the fit; sets the lower bound of the fitting range.
+     - No, from experiment/setup
+     - Instrumental
+   * - ``rmax``
+     - Maximum r (distance) included in the fit; sets the upper bound of the fitting range.
+     - No, from experiment/setup
+     - Instrumental (unless fitting to nanoparticles)

doc/source/snippets/structure_params.rst

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+.. list-table::
+   :widths: 2 6 2 2
+   :header-rows: 1
+   :class: wrap-text
+
+   * - PDF Parameter
+     - Description
+     - Refineable?
+     - Structural or Instrumental?
+   * - ``a``, ``b``, ``c``
+     - Lattice parameters of the crystal structure.
+     - Yes
+     - Structural
+   * - ``alpha``, ``beta``, ``gamma``
+     - Angles between lattice vectors
+     - Yes
+     - Structural
+   * - ``delta1``
+     - Constant (r-independent) peak broadening parameter accounting for uniform broadening of PDF peaks due to static disorder or residual instrumental effects.
+     - Yes
+     - Both
+   * - ``delta2``
+     - Correlated motion parameter accounting for r-dependent broadening (increases with r) of PDF peaks due to correlated atomic motion.
+     - Yes
+     - Structural
+   * - ``biso`` or ``Uiso``
+     - Isotropic atomic displacement parameter (ADP); models thermal vibration/disorder for each atom type. (note: ``biso=8*pi^2*Uiso``)
+     - Yes
+     - Structural
+   * - ``uij``
+     - Anisotropic ADPs constrained by space group symmetry.
+     - Yes
+     - Structural
+   * - ``dscale``
+     - Correction factor for background or density mismatch.
+     - Sometimes; often fixed
+     - Instrumental
+   * - ``sratio``
+     - Ratio of coherent to incoherent scattering, used in some models.
+     - Sometimes
+     - Instrumental