update llvm prereqs

Signed-off-by: Jade Abraham <[email protected]>

doc/rst/usingchapel/prereqs.rst

@@ -34,7 +34,7 @@ for using Chapel:
 
     * C11 support, while not required, will enable faster atomic operations.
 
-  * CMake is available and ``cmake`` runs version 3.13.4 or later.
+  * CMake is available and ``cmake`` runs version 3.20 or later.
 
   * The LLVM backend is now the default and it is easiest to use it with a
     system-wide installation of LLVM and clang. On Mac OS X, LLVM 14 and
@@ -44,6 +44,8 @@ for using Chapel:
     use the bundled LLVM or disable LLVM support (see
     :ref:`readme-chplenv.CHPL_LLVM`).
 
+     * Using the LLVM backend with LLVM 16 or newer requires C++17 support.
+
 In addition, several optional components have additional requirements:
 
   * Python 3.7 or newer is required if you want to use chpldoc, c2chapel,
@@ -59,9 +61,6 @@ In addition, several optional components have additional requirements:
 
   * ``pkg-config`` is required for the ``mason system`` subcommands
 
-  * ``cmake`` 3.16 or newer is required to install ``chpl`` when choosing an
-    installation with ``./configure --chpl-home=/path/to/install``
-
 
 .. _readme-prereqs-installation: