[docs] Fix doc formatting. (#2778)

Remove duplicated MT4171/MT4172 documentation, and fix some formatting to be proper markdown.

docs/website/mtouch-errors.md

@@ -1201,7 +1201,7 @@ Something unexpected occured when trying to mark `NSObject` subclasses from the
 
 Something unexpected occured when trying to inline code from the application. The assembly causing the issue is named in the error message. In order to fix this issue the assembly will need to be provided in a [bug report](https://bugzilla.xamarin.com) along with a complete build log with verbosity enabled (i.e. `-v -v -v -v` in the **Additional mtouch arguments**).
 
-<h3><a name="MT2100"/>MT2100: Smart Enum Conversion Preserver failed processing `...`.</h3>
+### <a name="MT2100"/>MT2100: Smart Enum Conversion Preserver failed processing `...`.
 
 Something unexpected occured when trying to mark the conversion methods for smart enums from the application. The assembly causing the issue is named in the error message. In order to fix this issue the assembly will need to be provided in a [bug report](https://bugzilla.xamarin.com) along with a complete build log with verbosity enabled (i.e. `-v -v -v -v` in the **Additional mtouch arguments**).
 
@@ -1556,11 +1556,11 @@ Please use a valid Objective-C identifier.
 Xamarin.iOS failed to generate a P/Invoke wrapper function for the mentioned.
 Please check the reported error message for the underlying cause.
 
-### <a name="MT4170"/>MT4170: The registrar can't convert from '{managed type}' to '{native type}' for the return value in the method {method}.
+### <a name="MT4170"/>MT4170: The registrar can't convert from '{managed type}' to '{native type}' for the return value in the method {method}.
 
 See the description of error <a href="#MT4172">MT4172</a>.
 
-## <h3><a name="MT4171"/>MT4171: The BindAs attribute on the member {member} is invalid: the BindAs type {type} is different from the property type {type}.
+### <a name="MT4171"/>MT4171: The BindAs attribute on the member {member} is invalid: the BindAs type {type} is different from the property type {type}.
 
 Please make sure the type in the BindAs attribute matches the type of the member it's attached to.
 
@@ -1579,14 +1579,6 @@ with a test case and we'll evaluate it.
 [1]: https://bugzilla.xamarin.com/enter_bug.cgi?product=iOS
 [2]: https://bugzilla.xamarin.com/enter_bug.cgi?product=iOS&component=General&bug_severity=enhancement
 
-<h3><a name="MT4170"/>MT4170: The registrar can't convert from '{managed type}' to '{native type}' for the return value in the method {method}.</h3>
-
-See the description of error <a href="#MT4169">MT4169</a>.
-
-<h3><a name="MT4171"/>MT4171: The BindAs attribute on the member {member} is invalid: the BindAs type {type} is different from the property type {type}.</h3>
-
-Please make sure the type in the BindAs attribute matches the type of the member it's attached to.
-
 # MT5xxx: GCC and toolchain error messages
 
 ### MT51xx: Compilation
@@ -2278,7 +2270,7 @@ This indicates a bug in Xamarin.iOS. Please file a bug at [http://bugzilla.xamar
 
 This indicates a bug in Xamarin.iOS. Please file a bug at [http://bugzilla.xamarin.com](https://bugzilla.xamarin.com/enter_bug.cgi?product=iOS).
 
-<h3><a name="MT8024"/>MT8024: Could not find a valid extension type for the smart enum '{smart_type}'. Please file a bug at https://bugzilla.xamarin.com.</h3>
+### <a name="MT8024"/>MT8024: Could not find a valid extension type for the smart enum '{smart_type}'. Please file a bug at https://bugzilla.xamarin.com.
 
 This indicates a bug in Xamarin.iOS. Please file a bug at [http://bugzilla.xamarin.com](https://bugzilla.xamarin.com/enter_bug.cgi?product=iOS).