[Docs][CMake] Add LLVM_ENABLE_HTTPLIB to CMake options list

This patch adds LLVM_ENABLE_HTTPLIB to the list of CMake options to make
it more clear exactly what it does and also provide clarity on which
specific project it is referring to/installation.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D152060

llvm/docs/CMake.rst

@@ -489,6 +489,11 @@ enabled sub-projects. Nearly all of these variable names begin with
 **LLVM_ENABLE_EXPENSIVE_CHECKS**:BOOL
   Enable additional time/memory expensive checking. Defaults to OFF.
 
+**LLVM_ENABLE_HTTPLIB**:BOOL
+  Enables the optional cpp-httplib dependency which is used by llvm-debuginfod
+  to serve debug info over HTTP. `cpp-httplib <https://github.com/yhirose/cpp-httplib>`_
+  must be installed, or `httplib_ROOT` must be set. Defaults to OFF.
+
 **LLVM_ENABLE_FFI**:BOOL
   Indicates whether the LLVM Interpreter will be linked with the Foreign Function
   Interface library (libffi) in order to enable calling external functions.