cmd/cgo: document #include <> search path behaviour

cgo effectively prepends -I${SRCDIR} to the header include path of all
preambles it processes, so when an #include <> matches a header file
both in the source directory and also another include directory, the
local copy will be used in preference.

This behaviour is surprising but unfortunately also longstanding and
relied upon by packages in the wild, so the best we can do is to
document it.

Fixes #41059

Change-Id: If6d2818294b2bd94ea0fe5fd6ce77e54b3e167a6
Reviewed-on: https://go-review.googlesource.com/c/go/+/251758


Reviewed-by: Ian Lance Taylor <iant@golang.org>