[nfc][6.40] Fix description of builtin build options

[ferdymercury]

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Test Results

2 613 tests   2 613 ✅  1h 23m 45s ⏱️
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Results for commit f430c98.

[dpiparo]

Thanks for this PR, which is a clear improvement. I am not sure about the word internally: perhaps something like

• from an external builtin package
• from external source code
• from an externally provided source tarball (maybe the most accurate?)

Maybe adding something like "for backwards compatibility", too?

[ferdymercury]

Thanks for the review!

Wrt "internally": I copy-pasted the wording from other builtins such as openssl or xrootd.
Wrt "for backward compatibility": maybe not needed since we already have the warning in ROOT_FIND_REQUIRED_DEP macro?

Since this is a patch for 6.40, should we maybe reword all of them instead just in this master-PR ? #21596 and leave this one as-is ?

[dpiparo]

--> Yes, my bad.