release note snippet for user flags in sanity checks [skip ci]

docs/markdown/snippets/sanity-check.md

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+## Sanity checking compilers with user flags
+
+Sanity checks previously only used user-specified flags for cross compilers, but
+now do in all cases.
+
+All compilers meson might decide to use for the build are "sanity checked"
+before other tests are run. This usually involves building simple executable and
+trying to run it. Previously user flags (compilation and/or linking flags) were
+used for sanity checking cross compilers, but not native compilers.  This is
+because such flags might be essential for a cross binary to succeed, but usually
+aren't for a native compiler.
+
+In recent releases, there has been an effort to minimize the special-casing of
+cross or native builds so as to make building more predictable in less-tested
+cases. Since this the user flags are necessary for cross, but not harmful for
+native, it makes more sense to use them in all sanity checks than use them in no
+sanity checks, so this is what we now do.