[llvm] [docs] Update LLVM_ENABLE_ZLIB/LLVM_ENABLE_ZSTD (PR #65218)

[Fangrui Song via llvm-commits]

https://github.com/MaskRay updated https://github.com/llvm/llvm-project/pull/65218:

>From 5ad1ee19fc50ed0a3be8b5353e206dc581e88bc3 Mon Sep 17 00:00:00 2001
From: Fangrui Song <i at maskray.me>
Date: Sat, 2 Sep 2023 22:03:25 -0700
Subject: [PATCH] [docs] Update LLVM_ENABLE_ZLIB/LLVM_ENABLE_ZSTD

e6c7ed6d2164a0659fd9f6ee44f1375d301e3cad changed LLVM_ENABLE_ZLIB to a
tri-state. Update the doc. Add doc for LLVM_ENABLE_ZSTD.

Close #65201
---
 llvm/docs/CMake.rst | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/llvm/docs/CMake.rst b/llvm/docs/CMake.rst
index d04feda0a771c14..67f740447ed7bfb 100644
--- a/llvm/docs/CMake.rst
+++ b/llvm/docs/CMake.rst
@@ -600,9 +600,15 @@ enabled sub-projects. Nearly all of these variable names begin with
   If enabled, the Z3 constraint solver is activated for the Clang static analyzer.
   A recent version of the z3 library needs to be available on the system.
 
-**LLVM_ENABLE_ZLIB**:BOOL
-  Enable building with zlib to support compression/uncompression in LLVM tools.
-  Defaults to ON.
+**LLVM_ENABLE_ZLIB**:STRING
+  Used to decide if LLVM tools should support compression/decompression with
+  zlib. Allowed values are ``OFF``, ``ON`` (default, enable if zlib is found),
+  and ``FORCE_ON`` (error if zlib is not found).
+
+**LLVM_ENABLE_ZSTD**:STRING
+  Used to decide if LLVM tools should support compression/decompression with
+  zstd. Allowed values are ``OFF``, ``ON`` (default, enable if zstd is found),
+  and ``FORCE_ON`` (error if zstd is not found).
 
 **LLVM_EXPERIMENTAL_TARGETS_TO_BUILD**:STRING
   Semicolon-separated list of experimental targets to build and linked into