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

[aidengrossman]

Summary

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.

[phosek]

LGTM

llvm/docs/CMake.rst

 **LLVM_ENABLE_EH**:BOOL
   Build LLVM with exception-handling support. This is necessary if you wish to
   link against LLVM libraries and make use of C++ exceptions in your own code
   that need to propagate through LLVM code. Defaults to OFF.
 
 **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.
   If the library or its headers are installed in a custom
   location, you can also set the variables FFI_INCLUDE_DIR and
   FFI_LIBRARY_DIR to the directories where ffi.h and libffi.so can be found,
   respectively. Defaults to OFF.