Add section to sanitizer doc for -Zexternal-clangrt

After spending a week looking for answers to how to do the very thing
this flag lets me do, it felt appropriate to document it where I would've
expected it to be.

Author: ObsidianMinor

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

@@ -853,6 +853,18 @@ functionality][build-std].
 
 [build-std]: ../../cargo/reference/unstable.html#build-std
 
+# Working with other languages
+
+Sanitizers rely on compiler runtime libraries to function properly. Rust links
+in its own compiler runtime which might conflict with runtimes required by
+languages such as C++. Since Rust's runtime doesn't always contain the symbols
+required by C++ instrumented code, you might need to skip linking it so another
+runtime can be linked instead.
+
+A separate unstable option `-Zexternal-clangrt` can be used to make rustc skip
+linking the compiler runtime for the sanitizer. This will require you to link
+in an external runtime, such as from clang instead.
+
 # Build scripts and procedural macros
 
 Use of sanitizers together with build scripts and procedural macros is