docs: Document LLD's cache dir argument.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@304385 91177308-0d34-0410-b5e6-96231b3b80d8

docs/ThinLTO.rst

@@ -123,6 +123,8 @@ which currently must be enabled through a linker option.
   ``-Wl,-plugin-opt,cache-dir=/path/to/cache``
 - ld64 (support in clang 3.9 and Xcode 8):
   ``-Wl,-cache_path_lto,/path/to/cache``
+- lld (as of LLVM r296702):
+  ``-Wl,--thinlto-cache-dir=/path/to/cache``
 
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