Doc: unstable book - profile.md: improve wording

- mention `--target` flag excludes RUSTFLAGS passing to build scripts and proc macros

Author: jschwe

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

@@ -21,6 +21,7 @@ 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` apply to everything that cargo builds and runs during a build, including build scripts!
-To avoid this, pass a `RUSTC_WRAPPER` program to cargo that only adds the profiling flags to rustc for the specific 
-crates you want to profile.
+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.