Work around rustdoc errors on nightly

Cap lints at allow so that doc error warnings do not stop
doc generation.

Testing: ran cargo test, fargo doc —open and fargo test.

Change-Id: Id4876e80e5430637cedfee080a222e0bf4d44865
diff --git a/src/lib.rs b/src/lib.rs
index d851364..328e28d 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -464,6 +464,7 @@
         )
         .env("RUSTC", rustc_path(target_options)?.to_str().unwrap())
         .env("RUSTDOC", rustdoc_path(target_options)?.to_str().unwrap())
+        .env("RUSTDOCFLAGS", "--cap-lints allow -Z unstable-options")
         .env(
             "FUCHSIA_SHARED_ROOT",
             shared_libraries_path(target_options)?,