Fix inconsistent namespace formatting guidelines

Suggested formatting for namespaces in the developer guide is currently
inconsistent. This commit updates the developer guide and clang-format
configuration to consistently put "namespace" and opening/closing braces
on the same line. Example:

```c++
namespace boost {
namespace signals2 {
class connection;
} // namespace signals2
} // namespace boost
```

Currently the "Source code organization" section has an example like the one
above, but the "Coding style" section example and description put a newline
between the opening "namespace foo" and brace (but oddly no newline between
closing namespace and brace).

Avoiding newlines before namespace opening braces makes nested declarations
less verbose and also avoids asymmetry with closing braces. It's also a
common style used in other codebases:

* https://google.github.io/styleguide/cppguide.html#Namespaces
* https://developer.mozilla.org/en-US/docs/Mozilla/Developer_guide/Coding_Style#Classes
* https://llvm.org/docs/CodingStandards.html#namespace-indentation

src/.clang-format

@@ -12,7 +12,10 @@ AlwaysBreakBeforeMultilineStrings: false
 AlwaysBreakTemplateDeclarations: true
 BinPackParameters: false
 BreakBeforeBinaryOperators: false
-BreakBeforeBraces: Linux
+BreakBeforeBraces: Custom
+BraceWrapping:
+  AfterClass: true
+  AfterFunction: true
 BreakBeforeTernaryOperators: false
 BreakConstructorInitializersBeforeComma: false
 ColumnLimit:     0