C#: Improve diagnostic messages and add help link

- Reword diagnostic messages.
- Add help link to diagnostics.

modules/mono/build_scripts/build_assemblies.py

@@ -316,9 +316,35 @@ def generate_sdk_package_versions():
         f.write(props)
         f.close()
 
+    # Also write the versioned docs URL to a constant for the Source Generators.
+
+    constants = """namespace Godot.SourceGenerators
+{{
+    partial class Common
+    {{
+        public const string VersionDocsUrl = "https://docs.godotengine.org/en/{docs_branch}";
+    }}
+}}
+""".format(
+        **version_info
+    )
+
+    generators_dir = os.path.join(
+        dirname(script_path),
+        "editor",
+        "Godot.NET.Sdk",
+        "Godot.SourceGenerators",
+        "Generated",
+    )
+    os.makedirs(generators_dir, exist_ok=True)
+
+    with open(os.path.join(generators_dir, "Common.Constants.cs"), "w") as f:
+        f.write(constants)
+        f.close()
+
 
 def build_all(msbuild_tool, module_dir, output_dir, godot_platform, dev_debug, push_nupkgs_local, precision):
-    # Generate SdkPackageVersions.props
+    # Generate SdkPackageVersions.props and VersionDocsUrl constant
     generate_sdk_package_versions()
 
     # Godot API