Update all LLVM documentation mentioning runtimes in LLVM_ENABLE_PROJECTS

We are moving away from building the runtimes with LLVM_ENABLE_PROJECTS,
however the documentation was largely outdated. This commit updates all
the documentation I could find to use LLVM_ENABLE_RUNTIMES instead of
LLVM_ENABLE_PROJECTS for building runtimes.

Note that in the near future, libcxx, libcxxabi and libunwind will stop
supporting being built with LLVM_ENABLE_PROJECTS altogether. I don't know
what the plans are for other runtimes like libc, openmp and compiler-rt,
so I didn't make any changes to the documentation that would imply
something for those projects.

Once this lands, I will also cherry-pick this on the release/14.x branch
to make sure that LLVM's documentation is up-to-date and reflects what
we intend to support in the future.

Differential Revision: https://reviews.llvm.org/D119351

NOKEYCHECK=True
GitOrigin-RevId: 4ae83bb2b1e27628f6bd14d746aef964ed776d52
diff --git a/docs/BuildingLibcxx.rst b/docs/BuildingLibcxx.rst
index 8c0d007..0e536cc 100644
--- a/docs/BuildingLibcxx.rst
+++ b/docs/BuildingLibcxx.rst
@@ -523,8 +523,8 @@
 
 .. code-block:: bash
 
-  $ cmake -G Ninja -S llvm -B build           \
-    -DLLVM_ENABLE_PROJECTS="libcxx"           \
+  $ cmake -G Ninja -S runtimes -B build       \
+    -DLLVM_ENABLE_RUNTIMES="libcxx"           \
     -DLIBCXX_CXX_ABI=libstdc++                \
     -DLIBCXX_CXX_ABI_INCLUDE_PATHS="/usr/include/c++/4.7/;/usr/include/c++/4.7/x86_64-linux-gnu/"
   $ ninja -C build install-cxx
@@ -549,8 +549,8 @@
 
 .. code-block:: bash
 
-  $ cmake -G Ninja -S llvm -B build                                   \
-          -DLLVM_ENABLE_PROJECTS="libcxx"                             \
+  $ cmake -G Ninja -S runtimes -B build                               \
+          -DLLVM_ENABLE_RUNTIMES="libcxx"                             \
           -DLIBCXX_CXX_ABI=libcxxrt                                   \
           -DLIBCXX_CXX_ABI_INCLUDE_PATHS=path/to/libcxxrt-sources/src
   $ ninja -C build install-cxx