[llvm][docs] Correct description of %t lit substitution (#164397)

%t is currently documented as:
temporary file name unique to the test

https://llvm.org/docs/CommandGuide/lit.html#substitutions

Which I take to mean if the path is a/b/c/tempfile, then %t would be
tempfile. It is not, it's the whole path.

(which is hinted at by %basename_t, but why would you read that if you
didn't need to use it)

As seen in #164396 this can create confusion when people use it as if it
were just the file name.

Make it clear in the docs that this is a unique path, which can be used
to make files or folders.

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