docs: Add documentation for LLVM_ENABLE_LIBEDIT CMake flag (#147112)
This adds missing documentation for the LLVM_ENABLE_LIBEDIT flag to the
CMake documentation page, addressing the issue that this flag was
undocumented despite being used in the build system.
The documentation explains:
- Purpose: Controls libedit support for command-line editing
- Supported values: ON, OFF, or auto-detection
- Default behavior: Auto-detection when not specified
Fixes: #146477