Add sections about panics during constant evaluation

Since const panics are now [stabilized], I figure it would be good to
document some of its gotchas. I couldn't really find documents for
the specifics I ran into. I am no const eval expert, and what I
wrote is basically a paraphrase of Ralf's comments[^1][^2],
but I hope to get the ball rolling for adding some docs.

I deliberately chose to not mention the guarantee about syntactically
referenced constants in functions that require code generation as it
seems hard to explain completely without some ambiguity. It also seems
to me that the rules are not completely stable[^3][^4] yet.

[stabilized]: rust-lang/rust#89006
[^1]: rust-lang/rust#91877 (comment)
[^2]: rust-lang/rust#91877 (comment)
[^3]: rust-lang/rust#71800
[^4]: rust-lang/rust#51999 (comment)

src/const_eval.md

@@ -99,6 +99,21 @@ Conversely, the following are possible in a const function, but not in a const c
 * Use of generic type and lifetime parameters.
   * Const contexts do allow limited use of [const generic parameters].
 
+## Panics
+
+Compile time evaluation of constant expressions that that panic can cause
+compilation errors.
+
+## Evaluation timing
+
+Not all const contexts that appear in the source code necessarily trigger
+compile time constant evaluation. For example, the compiler can only evaluate
+constant expressions that use [const generic parameters] once concrete values are
+known.
+
+Top level constants are guaranteed to be evaluated at compile time, even when
+the constants are unused.
+
 [arithmetic]:           expressions/operator-expr.md#arithmetic-and-logical-binary-operators
 [array expressions]:    expressions/array-expr.md
 [array indexing]:       expressions/array-expr.md#array-and-slice-indexing-expressions