possible out of date documentation for `-Zdump-solver-proof-tree`

[GTimothy]

The documentation for proof trees seems out of date.
In rustc-dev-guide/src/solve/proof-trees.md, line 19 there is a mention

For debugging you can use -Zdump-solver-proof-tree which dumps the proof tree
for all goals proven by the trait solver in the current session.

This documentation was added by @lcnr on Jun 23, 2023, 4 days after the corresponding merge commit in rust-lang/rust.

But it seems this code was then modified/renamed/deleted by @lcnr on December 14, 2023, see this commit.

I am unsure what replacement flag (if there is any) should be documented. But -Zdump-solver-proof-tree is not a working flag anymore.

At least not on nightly-x86_64-unknown-linux-gnu - rustc 1.82.0-nightly (515395af0 2024-08-26):

error: unknown unstable option: dump-solver-proof-tree