[docs] Update LLVM_ENABLE_ZLIB/LLVM_ENABLE_ZSTD (#65218)

e6c7ed6 changed LLVM_ENABLE_ZLIB to a
tri-state. Update the doc. Add doc for LLVM_ENABLE_ZSTD.

Close #65201

Author: MaskRay

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