message: [c#] Improve codegen targets comments
diff: </BondImportDirectory>
  </ItemDefinitionGroup>
  + <!-- Let Visual Studio know about the BondCodegen item so it can display
  + it in the UI as a potential action for files. -->
  <ItemGroup>
  <AvailableItemName Include="BondCodegen" />
  </ItemGroup>
  <UpToDateCheckInput Include="$(_BondExe)" />
  </ItemGroup>
  - <!--
  -
  - * BondCodegenCs - transform .bond files into .cs files
  - * If we use output transforms from BondCodegen to the resulting targets we can get partial/minimal rebuilds
  - * on change, but this doesn't reflect the internal import logic of bond. A change to common.bond would
  - * only rebuild common*cs, not all the others that import common.bond. Without a bond parser prepass we
  - * must rebuild all .bond files when any of them change. We force that by putting a non-transform output
  - * (bondcodegen.done) in the list so IFF any input changes, then all inputs rebuild since MSBuild can't
  - * know what should create the unmapped output item and so can't be selective.
  - * NOTE - this still won't catch changes to files outside declared BondCodegen elements.
  - -->
  + <!-- BondCodegenCs - transform .bond files into .cs files
  +
  + If we were to use an output transform from BondCodegen to the
  + resulting .cs files we could get partial/minimal rebuilds on changes,
  + but this behavior doesn't reflect the internal import logic of Bond.
  + A change to common.bond would only rebuild common*cs, not all the
  + other files that import common.bond. Without a Bond parser prepass we
  + must rebuild all .bond files when any of them change. We force that
  + by putting a non-transform output (bondcodegen.done) in the list so
  + if any input changes, then all inputs rebuild since MSBuild can't
  + know what should create the unmapped output item and so can't be
  + selective.
  +
  + NOTE: This still won't catch changes to files imported outside the
  + BondCodegen item. -->
  <Target Name="BondCodegenCs"
  Inputs="$(_BondExe);@(BondCodegen)"
  Outputs="$(BondOutputDirectory)\bondcodegen.done;@(BondCodegen -> '$(BondOutputDirectory)\%(FileName)_types.cs')"
  