docs: update mtsat docs

Docs/MT.rst

@@ -249,7 +249,7 @@ If you use a custom contrasts file then the outputs will have the names specifie
 qi mtsat
 -----------
 
-Implementation of Gunther Helm's MT-Sat method. Calculates R1, apparent PD and the semi-quantitative MT-Saturation parameter "delta". This is the fractional reduction in the longitudinal magnetization during one TR, expressed as a percentage. Arguably could be included in the :doc:`Relaxometry` module instead. Outputs R1 instead of T1 as this is more common in the MTSat / MPM literature. If using multi-echo input data the input should be passed through `qi mpm_r2s` first and the output ``S0`` files used as input to `qi mtsat`. Incorporate the removal of the small flip angle approximation for PDw and T1w images, as suggested in Edwards et al., 2021.
+Implementation of Gunther Helm's MT-Sat method. Calculates R1, apparent PD and the semi-quantitative MT-Saturation parameter "delta". This is the fractional reduction in the longitudinal magnetization during one TR, expressed as a percentage. Arguably could be included in the :doc:`Relaxometry` module instead. Outputs R1 instead of T1 as this is more common in the MTSat / MPM literature. If using multi-echo input data the input should be passed through `qi mpm_r2s` first and the output ``S0`` files used as input to `qi mtsat`. By default, does not use the original small flip angle approximation to compute R1 and PD because of the results presented in Edwards et al., 2021. To still use the small flip angle approximation (e.g., for backward compatibility), use the --smallangle (-s) flag.
 
 **Example Command Line**
 
@@ -278,6 +278,12 @@ Implementation of Gunther Helm's MT-Sat method. Calculates R1, apparent PD and t
 - ``MTSat_S0.nii.gz`` - Apparent proton density / equilibrium magnetization
 - ``MTSat_delta.nii.gz`` - MT-Sat parameter, see above.
 
+*Important Options*
+
+* ``--smallangle, -s``
+
+    If this flag is specified, use the small flip angle approximation from the original Helms et al paper. Else, do not use the approximation, as suggested in Edwards et al (default).
+
 **References**
 
 - `Helms et al <http://doi.wiley.com/10.1002/mrm.21732>`_