Documented specific checks/subchecks for header file verification
Simplified the process, now we use simple regex to remove macros from
the current header. Before, we re-ran preprocess_one_header()
function with removed `#include ...`s from the header under test, so
we were looking into the actual header (rather than included headers)
when checking for `extern "C"` keyword.
The procedure is easier to follow without this recursion (mostly because
in the second execution we might encounter compilation failers and
ignore them).
Note that this procedure is not 100% correct (we might see some false
positive and false negatives)
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