docs: one side strict property mappings (#810)

docs/docs/configuration/mapper.mdx

@@ -182,6 +182,49 @@ dotnet_diagnostic.RMG012.severity = error # Unmapped target member
 dotnet_diagnostic.RMG020.severity = error # Unmapped source member
 ```
 
+#### One side strict property mappings
+
+To enforce strict mappings on only either the source or the target,
+the `RequiredMappingStrategy` can be used.
+
+<!-- do not indent this, it won't work, https://stackoverflow.com/a/67579641/3302887 -->
+
+<Tabs>
+<TabItem value="global" label="Global (mapper level)" default>
+
+Sets the `RequiredMappingStrategy` for all methods within the mapper,
+by default it is `Both` requiring all members to be mapped.
+This can be overriden by individual mapping methods using `MapperRequiredMappingAttribute`.
+
+```csharp
+// highlight-start
+[Mapper(RequiredMappingStrategy = RequiredMappingStrategy.Source)]
+// highlight-end
+public partial class CarMapper
+{
+  ...
+}
+```
+
+</TabItem>
+<TabItem value="enum" label="Enum (mapping method level)">
+
+Applied to the specific mapping method.
+
+```csharp
+[Mapper]
+public partial class CarMapper
+{
+  // highlight-start
+  [MapperRequiredMapping(RequiredMappingStrategy.Source)]
+  // highlight-end
+  public partial CarDto MapMake(Car make);
+}
+```
+
+</TabItem>
+</Tabs>
+
 ### Strict enum mappings
 
 To enforce strict enum mappings set `RMG037` and `RMG038` to error, see [strict enum mappings](./enum.mdx#strict-enum-mappings).