[libc++] Update contributor documentation.

The document has the following updates:
- Rename 'feature test' to 'feature-test', the latter is the spelling
  used in the Standard.
- Add information how an ABI list can be downloaded from Buildkite.

Differential Revision: https://reviews.llvm.org/D99290

diff --git a/libcxx/docs/Contributing.rst b/libcxx/docs/Contributing.rst
index de71c7e..6bad6a6 100644 (file)
--- a/libcxx/docs/Contributing.rst
+++ b/libcxx/docs/Contributing.rst
@@ -33,11 +33,16 @@ After branching for an LLVM release:
 2. Update the ``include/__libcpp_version`` file
 3. Update the version number in ``docs/conf.py``
 
-Modifying feature test macros
+Modifying feature-test macros
 =============================
 
-When adding or updating feature test macros, you should update the corresponding tests.
-To do that, modify ``feature_test_macros`` table in the script ``utils/generate_feature_test_macro_components.py``, run it, and commit updated files.
+When adding or updating feature-test macros, you should update the corresponding tests.
+To do that, modify ``feature_test_macros`` table in the script
+``utils/generate_feature_test_macro_components.py``, run it, and commit updated
+files. Running ``utils/generate_feature_test_macro_components.py`` should never
+generate diffs in a clean checkout; feel free to run it in your local checkout
+any time you want.
+
 
 Adding a new header TODO
 ========================
@@ -53,6 +58,15 @@ When adding a new header to libc++:
 Exporting new symbols from the library
 ======================================
 
-When exporting new symbols from libc++, one must update the ABI lists located in ``lib/abi``.
+When exporting new symbols from libc++, you must update the ABI lists located in ``lib/abi``.
 To test whether the lists are up-to-date, please run the target ``check-cxx-abilist``.
 To regenerate the lists, use the target ``generate-cxx-abilist``.
+The ABI lists must be updated for all supported platforms; currently Linux and
+Apple.  If you don't have access to one of these platforms, you can download an
+updated list from the failed build at
+`Buildkite <https://buildkite.com/llvm-project/libcxx-ci>`__.
+Look for the failed build and select the ``artifacts`` tab. There, download the
+abilist for the platform, e.g.:
+
+* C++20 for the Linux platform.
+* MacOS C++20 for the Apple platform.