doc: add --test-builder/--test-builder-wrapper

Author: tmfink

src/doc/rustdoc/src/command-line-arguments.md

@@ -427,3 +427,32 @@ This flag is **deprecated** and **has no effect**.
 Rustdoc only supports Rust source code and Markdown input formats. If the
 file ends in `.md` or `.markdown`, `rustdoc` treats it as a Markdown file.
 Otherwise, it assumes that the input file is Rust.
+
+## `--test-builder`: `rustc`-like program to build tests
+
+Using this flag looks like this:
+
+```bash
+$ rustdoc --test-builder /path/to/rustc src/lib.rs
+```
+
+Rustdoc will use the provided program to compile tests instead of the default `rustc` program from
+the sysroot.
+
+## `--test-builder-wrapper`: wrap calls to the test builder
+
+Using this flag looks like this:
+
+```bash
+$ rustdoc --test-builder-wrapper /path/to/rustc-wrapper src/lib.rs
+$ rustdoc \
+    --test-builder-wrapper rustc-wrapper1 \
+    --test-builder-wrapper rustc-wrapper2 \
+    --test-builder rustc \
+    src/lib.rs
+```
+
+Similar to cargo `build.rustc-wrapper` option, this flag takes a `rustc` wrapper program.
+The first argument to the program will be the test builder program.
+
+This flag can be passed multiple times to nest wrappers.