[docs] Mention LLVM_USE_LINKER/LLVM_PARALLEL_{COMPILE,LINK}_JOBS in GettingStarted

[aeubanks]

Summary

As requested in D145413.

[aaron.ballman]

LGTM, thank you!

llvm/docs/GettingStarted.rst

* ``cmake -S llvm -B build -G <generator> [options]``
      * ``-DCMAKE_BUILD_TYPE=type`` --- Controls optimization level and debug
        information of the build. Valid options for *type* are ``Debug``,
        ``Release``, ``RelWithDebInfo``, and ``MinSizeRel``. For more detailed
        information see :ref:`CMAKE_BUILD_TYPE <cmake_build_type>`.
 
      * ``-DLLVM_ENABLE_ASSERTIONS=ON`` --- Compile with assertion checks enabled
        (default is ON for Debug builds, OFF for all other build types).
 
+     * ``-DLLVM_USE_LINKER=lld`` --- Link with the `lld linker`_, assuming it
+       is installed on your system. This can dramatically speed up link times
+       if the default linker is slow.
+
+     * ``-DLLVM_PARALLEL_{COMPILE,LINK}_JOBS=N`` --- Limit the number of
+       compile/link jobs running in parallel at the same time. This is
+       especially important for linking since linking can use lots of memory. If
+       you run into memory issues building LLVM, try setting this to limit the
+       maximum number of compile/link jobs running at the same time.
+
    * ``cmake --build . [--target <target>]`` or the build system specified
      above directly.
 
      * The default target (i.e. ``cmake --build .`` or ``make``) will build all of
        LLVM.
 
      * The ``check-all`` target (i.e. ``ninja check-all``) will run the
        regression tests to ensure everything is in working order.