Fix documentation example for gcov profiling

src/doc/unstable-book/src/compiler-flags/profile.md

@@ -12,10 +12,16 @@ For example:
 ```Bash
 cargo new testgcov --bin
 cd testgcov
-export RUSTFLAGS="-Zprofile"
+export RUSTFLAGS="-Zprofile -Ccodegen-units=1 -Copt-level=0 -Clink-dead-code -Coverflow-checks=off -Zpanic_abort_tests -Cpanic=abort"
+export CARGO_INCREMENTAL=0
 cargo build
 cargo run
 ```
 
 Once you've built and run your program, files with the `gcno` (after build) and `gcda` (after execution) extensions will be created.
 You can parse them with [llvm-cov gcov](https://llvm.org/docs/CommandGuide/llvm-cov.html#llvm-cov-gcov) or [grcov](https://github.com/mozilla/grcov).
+
+Please note that `RUSTFLAGS` by default applies to everything that cargo builds and runs during a build!
+When the `--target` flag is explicitly passed to cargo, the `RUSTFLAGS` no longer apply to build scripts and procedural macros. 
+For more fine-grained control consider passing a `RUSTC_WRAPPER` program to cargo that only adds the profiling flags to 
+rustc for the specific crates you want to profile.