docs: Add documentation for LLVM_ENABLE_LIBEDIT CMake flag

[toprakmurat]

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

[github-actions]

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[yashssh]

Thanks for the PR! It lgtm but please wait for couple of days.