Permit nested using without indentation (#31816)

Author: Neal Gafter

Documentation/coding-guidelines/coding-style.md

@@ -7,7 +7,7 @@ For non code files (xml, etc), our current best guidance is consistency. When ed
 
 The general rule we follow is "use Visual Studio defaults".
 
-1. We use [Allman style](http://en.wikipedia.org/wiki/Indent_style#Allman_style) braces, where each brace begins on a new line. A single line statement block can go without braces but the block must be properly indented on its own line and must not be nested in other statement blocks that use braces (See issue [381](https://github.com/dotnet/corefx/issues/381) for examples). 
+1. We use [Allman style](http://en.wikipedia.org/wiki/Indent_style#Allman_style) braces, where each brace begins on a new line. A single line statement block can go without braces but the block must be properly indented on its own line and must not be nested in other statement blocks that use braces (See issue [381](https://github.com/dotnet/corefx/issues/381) for examples). One exception is that a `using` statement is permitted to be nested within another `using` statement by starting on the following line at the same indentation level, even if the nested `using` contains a controlled block.
 2. We use four spaces of indentation (no tabs).
 3. We use `_camelCase` for internal and private fields and use `readonly` where possible. Prefix internal and private instance fields with `_`, static fields with `s_` and thread static fields with `t_`. When used on static fields, `readonly` should come after `static` (e.g. `static readonly` not `readonly static`).  Public fields should be used sparingly and should use PascalCasing with no prefix when used.
 4. We avoid `this.` unless absolutely necessary.