docs(gga_grad): sync parameters.yaml and document gga_grad in input-main.md

Add item.description for gga_grad in read_input_item_elec_stru.cpp and
regenerate docs/parameters.yaml to fix the CI check that compares the
yaml against the output of --generate-parameters-yaml. Also add the
gga_grad entry to docs/advanced/input_files/input-main.md.

Author: dyzheng

docs/advanced/input_files/input-main.md

@@ -1268,6 +1268,17 @@
   - 4: For the case of noncollinear polarized, nspin will be automatically set to 4 without being specified by the user.
 - **Default**: 1
 
+### gga_grad
+
+- **Type**: Integer
+- **Description**: Method used to evaluate the density gradient entering GGA exchange-correlation terms in noncollinear-spin (nspin=4) calculations.
+  - 1: collinear approximation, only the gradient of the magnetization magnitude |m| is used.
+  - 2: projected method, the gradient of the magnetization direction is projected out via div(h), with h = m/|m|.
+  - 3: Scalmani-Frisch transformation (G. Scalmani and M. J. Frisch, J. Chem. Theory Comput. 8, 2193 (2012)), which retains all cross terms of grad(m/|m|); the most accurate and the default.
+
+  This parameter only takes effect for nspin=4 with GGA functionals (and magnetic calculation).
+- **Default**: 3
+
 ### smearing_method
 
 - **Type**: String

docs/parameters.yaml

@@ -665,6 +665,18 @@ parameters:
     default_value: "1"
     unit: ""
     availability: ""
+  - name: gga_grad
+    category: Electronic structure
+    type: Integer
+    description: |
+      Method used to evaluate the density gradient entering GGA exchange-correlation terms in noncollinear-spin (nspin=4) calculations.
+      * 1: collinear approximation, only the gradient of the magnetization magnitude |m| is used.
+      * 2: projected method, the gradient of the magnetization direction is projected out via div(h), with h = m/|m|.
+      * 3: Scalmani-Frisch transformation (G. Scalmani and M. J. Frisch, J. Chem. Theory Comput. 8, 2193 (2012)), which retains all cross terms of grad(m/|m|); the most accurate and the default.
+      This parameter only takes effect for nspin=4 with GGA functionals (and magnetic calculation).
+    default_value: "3"
+    unit: ""
+    availability: ""
   - name: smearing_method
     category: Electronic structure
     type: String

source/source_io/module_parameter/read_input_item_elec_stru.cpp

@@ -506,6 +506,11 @@ The other way is only available when compiling with LIBXC, and it allows for sup
         item.annotation = "GGA gradient method for nspin=4: 1 collinear approx, 2 projected div(h), 3 Scalmani-Frisch";
         item.category = "Electronic structure";
         item.type = "Integer";
+        item.description = R"(Method used to evaluate the density gradient entering GGA exchange-correlation terms in noncollinear-spin (nspin=4) calculations.
+* 1: collinear approximation, only the gradient of the magnetization magnitude |m| is used.
+* 2: projected method, the gradient of the magnetization direction is projected out via div(h), with h = m/|m|.
+* 3: Scalmani-Frisch transformation (G. Scalmani and M. J. Frisch, J. Chem. Theory Comput. 8, 2193 (2012)), which retains all cross terms of grad(m/|m|); the most accurate and the default.
+This parameter only takes effect for nspin=4 with GGA functionals (and magnetic calculation).)";
         item.default_value = "3";
         read_sync_int(input.gga_grad);
         item.check_value = [](const Input_Item& item, const Parameter& para) {