Update C# instructions in documentation

README.md

@@ -656,9 +656,13 @@ process.
 
     - C# support: install [Mono on non-Windows platforms][mono-install].
       Navigate to `YouCompleteMe/third_party/ycmd/third_party/OmniSharpServer`
-      and run `msbuild /property:Configuration=Release` on Windows. Replace
-      `msbuild` by `xbuild` on other platforms. On Windows, be sure that [the
-      build utility `msbuild` is in your PATH][add-msbuild-to-path].
+      and run
+
+          msbuild /property:Configuration=Release /property:TargetFrameworkVersion=v4.5
+
+      Replace `msbuild` by `xbuild` if `msbuild` is not available. On Windows,
+      be sure that [the build utility `msbuild` is in your
+      PATH][add-msbuild-to-path].
 
     - Go support: install [Go][go-install] and add it to your path. Navigate to
       `YouCompleteMe/third_party/ycmd/third_party/gocode` and run `go build`.

doc/youcompleteme.txt

@@ -870,9 +870,12 @@ will notify you to recompile it. You should then rerun the install process.
 
    - C# support: install Mono on non-Windows platforms [41]. Navigate to
      'YouCompleteMe/third_party/ycmd/third_party/OmniSharpServer' and run
-     'msbuild /property:Configuration=Release' on Windows. Replace
-     'msbuild' by 'xbuild' on other platforms. On Windows, be sure that
-     the build utility 'msbuild' is in your PATH [37].
+
+     msbuild /property:Configuration=Release
+     /property:TargetFrameworkVersion=v4.5
+
+   Replace 'msbuild' by 'xbuild' if 'msbuild' is not available. On Windows,
+   be sure that the build utility 'msbuild' is in your PATH [37].
 
    - Go support: install Go [27] and add it to your path. Navigate to
      'YouCompleteMe/third_party/ycmd/third_party/gocode' and run 'go