Rollup merge of #124488 - est31:arbitrary_expressions_error, r=pnkfelix

Add a note to the ArbitraryExpressionInPattern error

The current "arbitrary expressions aren't allowed in patterns" error is confusing, as it fires for code where it *looks* like a pattern but the compiler still treats it as an expression. That this is due to the `:expr` fragment specifier forcing the expression-ness property on the code.

In the test suite, the "arbitrary expressions aren't allowed in patterns" error can only be found in combination with macro_rules macros that force expression-ness of their content, namely via `:expr` metavariables. I also can't come up with cases where there would be an expression instead of a pattern, so I think it's always coming from an `:expr`.

In order to make the error less confusing, this adds a note explaining the weird `:expr` fragment behaviour.

Fixes #99380

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