Add check_header to Reference manual and release notes [skip ci]

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+## New compiler check: check_header()
+
+The existing compiler check `has_header()` only checks if the header exists,
+either with the `__has_include` C++11 builtin, or by running the pre-processor.
+
+However, sometimes the header you are looking for is unusable on some platforms
+or with some compilers in a way that is only detectable at compile-time. For
+such cases, you should use `check_header()` which will include the header and
+run a full compile.
+
+Note that `has_header()` is much faster than `check_header()`, so it should be
+used whenever possible.