Rollup merge of rust-lang#74522 - tmiasko:sanitizer-docs, r=nikomatsakis

Update sanitizer docs

* Document AddressSanitizer memory leak detection defaults.
* Remove CC & CFLAGS from MemorySanitizer example - they are now unnecessary for pure Rust projects (backtrace-rs moved away from libbacktrace).

Author: Manishearth

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

@@ -26,6 +26,9 @@ of bugs:
 * Double-free, invalid free
 * Memory leaks
 
+The memory leak detection is enabled by default on Linux, and can be enabled
+with runtime flag `ASAN_OPTIONS=detect_leaks=1` on macOS.
+
 AddressSanitizer is supported on the following targets:
 
 * `x86_64-apple-darwin`
@@ -196,10 +199,6 @@ fn main() {
 
 ```shell
 $ export \
-  CC=clang \
-  CXX=clang++ \
-  CFLAGS='-fsanitize=memory -fsanitize-memory-track-origins' \
-  CXXFLAGS='-fsanitize=memory -fsanitize-memory-track-origins' \
   RUSTFLAGS='-Zsanitizer=memory -Zsanitizer-memory-track-origins' \
   RUSTDOCFLAGS='-Zsanitizer=memory -Zsanitizer-memory-track-origins'
 $ cargo clean