fix: toolchain - update docs

toolchain/build-montauk-toolchain.sh

@@ -121,8 +121,12 @@ cp -r "${REPO_DIR}/programs/include/libc/." "${SYSROOT}/usr/include/"
 cp -r "${REPO_DIR}/programs/include/montauk" "${SYSROOT}/usr/include/montauk"
 cp -r "${REPO_DIR}/programs/include/Api" "${SYSROOT}/usr/include/Api"
 # Freestanding C++ headers (<cstdint>, <type_traits>, <atomic>, ...) —
-# there is no hosted libstdc++ yet, so C++ programs get the same
-# freestanding subset the rest of the tree builds against.
+# the subset the kernel and the in-tree programs build against. This is
+# NOT the whole C++ story: a hosted libstdc++ is built further down and
+# installs its own headers under ${PREFIX}/x86_64-montauk/include/c++.
+# That directory comes first on the include path and this one comes
+# last, so a plain g++ invocation gets the hosted headers and these are
+# shadowed. See "C++ standard library" in README.md.
 cp -r "${REPO_DIR}/kernel/freestnd-cxx-hdrs/x86_64/include/." "${SYSROOT}/usr/include/"
 cp "${REPO_DIR}/programs/lib/libc/crt1.o" \
    "${REPO_DIR}/programs/lib/libc/crti.o" \
@@ -161,9 +165,11 @@ else
 fi
 
 # ── Build GCC ─────────────────────────────────────────────────────────────────
-# Hosted against the Montauk libc headers in the sysroot. Only the
-# compiler and libgcc are built; libstdc++ needs a more complete libc
-# (that is the self-hosting step, not the cross-toolchain step).
+# Hosted against the Montauk libc headers in the sysroot. Builds the
+# compiler, libgcc, and a hosted libstdc++ (full STL: string/vector/map,
+# exceptions, RTTI, iostream). libstdc++ is built gthr-single because
+# the libc has no pthreads, so <thread>/<mutex> are absent and
+# shared_ptr uses non-atomic refcounts. See README.md.
 if [[ -x "${PREFIX}/bin/${TARGET}-gcc" ]]; then
     bold "GCC already installed — skipping."
 else