Update Omnisharp instructions in documentation

The /property:Configuration=Release option is not mentioned when
building the OmniSharp server in the full installation guide.

README.md

@@ -638,9 +638,9 @@ process.
 
   - C# support: install [Mono on non-Windows platforms][mono-install]. Navigate
     to `YouCompleteMe/third_party/ycmd/third_party/OmniSharpServer` and run
-    `msbuild` (Windows) or `xbuild` (other platforms, using Mono) depending on
-    your platform. On Windows, be sure that [the build utility `msbuild` is in
-    your PATH][add-msbuild-to-path].
+    `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].
 
   - 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

@@ -855,9 +855,9 @@ will notify you to recompile it. You should then rerun the install process.
 
 6. C# support: install Mono on non-Windows platforms [37]. Navigate to
    'YouCompleteMe/third_party/ycmd/third_party/OmniSharpServer' and run
-   'msbuild' (Windows) or 'xbuild' (other platforms, using Mono) depending
-   on your platform. On Windows, be sure that the build utility 'msbuild' is
-   in your PATH [33].
+   '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 [33].
 
 7. Go support: install Go [23] and add it to your path. Navigate to
    'YouCompleteMe/third_party/ycmd/third_party/gocode' and run 'go build'.