Document -C symbol-mangling-version

Author: ehuss

src/doc/rustc/src/codegen-options/index.md

@@ -541,6 +541,21 @@ Supported values for this option are:
 - `symbols` - same as `debuginfo`, but the rest of the symbol table section is
   stripped as well if the linker supports it.
 
+## symbol-mangling-version
+
+This option controls the [name mangling] format for encoding Rust item names
+for the purpose of generating object code and linking.
+
+Supported values for this option are:
+
+* `v0` — The "v0" mangling scheme. The specific format is not specified at
+  this time.
+
+The default if not specified will use a compiler-chosen default which may
+change in the future.
+
+[name mangling]: https://en.wikipedia.org/wiki/Name_mangling
+
 ## target-cpu
 
 This instructs `rustc` to generate code specifically for a particular processor.