Update Fuchsia walkthrough with new configs

The new `download-ci-llvm` configuration option dosn't work with `lld =
true` (see rust-lang#100853). The Fuchsia walkthrough should recommend setting it
to `false`.

Author: djkoloski

src/doc/rustc/src/platform-support/fuchsia.md

@@ -198,6 +198,9 @@ target = ["<host_platform>", "aarch64-fuchsia", "x86_64-fuchsia"]
 [rust]
 lld = true
 
+[llvm]
+download-ci-llvm = false
+
 [target.x86_64-fuchsia]
 cc = "clang"
 cxx = "clang++"