doc: clarify IWYU workflow

Document the CI wrapper as the reproducible IWYU entrypoint instead of suggesting ad hoc native runs.
Also describe how to handle suspected false positives, explain when local `IWYU pragma` workarounds are appropriate, and add an example rationale to an existing pragma.

Co-authored-by: Hennadii Stepanov <32963518+hebasto@users.noreply.github.com>
Co-authored-by: Daniel Pfeifer <daniel@pfeifer-mail.de>

src/node/blockstorage.h

@@ -18,6 +18,8 @@
 #include <streams.h>
 #include <sync.h>
 #include <uint256.h>
+// IWYU incorrectly suggests removing this header.
+// See https://github.com/include-what-you-use/include-what-you-use/issues/2014.
 #include <util/byte_units.h> // IWYU pragma: keep
 #include <util/expected.h>
 #include <util/fs.h>