Add markup for libc++ dylib availability

Libc++ is used as a system library on macOS and iOS (amongst others). In order
for users to be able to compile a binary that is intended to be deployed to an
older version of the platform, clang provides the
availability attribute <https://clang.llvm.org/docs/AttributeReference.html#availability>_
that can be placed on declarations to describe the lifecycle of a symbol in the
library.

See docs/DesignDocs/AvailabilityMarkup.rst for more information.

Differential Revision: https://reviews.llvm.org/D31739

llvm-svn: 302172
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: e9c66ad9fa9f1c1138c2a4868610e1d77271216c
diff --git a/docs/index.rst b/docs/index.rst
index c4cdf3b..3526b47 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -128,6 +128,7 @@
 .. toctree::
    :maxdepth: 1
 
+   DesignDocs/AvailabilityMarkup
    DesignDocs/DebugMode
    DesignDocs/CapturingConfigInfo
    DesignDocs/ABIVersioning