docs/20: noun-phrase headings, which the style check requires

`Is the extern "C" header required?` and its Chinese twin were question
headings. The check names the rule and both files; the content is unchanged.

Author: speak-agent

docs/20-heterogeneous-builds.md

@@ -147,7 +147,7 @@ links libstdc++ puts a second copy of the C++ runtime into a program whose own
 copy came from mcpp's toolchain.
 
 
-#### Is the `extern "C"` header required?
+#### The `extern "C"` header, and what omitting it costs
 
 Not by the language. The seam can declare the entry point itself and the island
 can define it, with no header anywhere, and that builds and links and runs.

docs/zh/20-heterogeneous-builds.md

@@ -118,7 +118,7 @@ tarball 已经发出去了。设备源文件必须被显式点名。
 自身运行时来自 mcpp 工具链的程序里。
 
 
-#### 那个 `extern "C"` 头文件是必须的吗?
+#### 那个 `extern "C"` 头文件,以及省掉它的代价
 
 语言上不是。接缝可以自己声明入口点、岛自己定义它,一个头文件都不写,照样编得过、链得上、
 跑得起来。