Add documentation for {{rust-src-base}} (#2079)

Author: Urgau

Diff for: src/tests/headers.md

@@ -285,6 +285,7 @@ described below:
 - `{{build-base}}`: The base directory where the test's output goes. This is
   equivalent to `$TEST_BUILD_DIR` for [output normalization].
   - Example: `/path/to/rust/build/x86_64-unknown-linux-gnu/test/ui`
+- `{{rust-src-base}}`: The sysroot directory where libstd/libcore/... are located
 - `{{sysroot-base}}`: Path of the sysroot directory used to build the test.
   - Mainly intended for `ui-fulldeps` tests that run the compiler via API.
 - `{{target-linker}}`: Linker that would be passed to `-Clinker` for this test,