[libc++][doc] Add warning about limitation of --fresh in boostrapping build (#65265)

Add a warning to the `Building Libcxx` documentation about the
limitations of the utility of `--fresh` at the top level.

Signed-off-by: Will Hawkins <[email protected]>

Author: hawkinsw

libcxx/docs/BuildingLibcxx.rst

@@ -78,6 +78,10 @@ CMake invocation at ``<monorepo>/llvm``:
   This type of build is also commonly called a "Runtimes build", but we would like to move
   away from that terminology, which is too confusing.
 
+.. warning::
+  Adding the `--fresh` flag to the top-level cmake invocation in a bootstrapping build *will not*
+  freshen the cmake cache of any of the enabled runtimes.
+
 Support for Windows
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