What the header check can actually contradict, said in headers too

BUILT.md and the web page both make the point that the generated half agrees
with the header by construction; headers said "425 declarations" and left a
reader to infer that all 425 are independently verified. It is the file
somebody reads when deciding whether to export the variable at all.

vendor/raylib/headers

@@ -17,9 +17,12 @@
 #   2. an ordinary build, when the variable happens to be set. Every C symbol
 #      is bound already — by hand in raylib.flan or by generation in
 #      generated.flan — so the importer generates nothing and the header read
-#      is purely the check. That check is now over all 425 declarations rather
-#      than the 172 hand-written ones, because the generated file is a package
-#      file like any other and is checked like one.
+#      is purely the check. It runs over all 425 declarations rather than the
+#      172 hand-written ones, because the generated file is a package file like
+#      any other and is checked like one — though only the hand-written ones can
+#      actually disagree, since the generated half came out of this header and
+#      agrees with it by construction. That is also why the hand-written lines
+#      were kept rather than replaced by generated ones.
 #
 # Why it is still optional. A build needs libraylib linkable and *not*
 # raylib-devel installed, which is a property worth keeping; requiring a header