Rollup merge of #87715 - bhgomes:long-explanation-E0625, r=GuillaumeGomez

Add long error explanation for E0625

For #61137.

compiler/rustc_error_codes/src/error_codes/E0625.md

@@ -0,0 +1,28 @@
+A compile-time const variable is referring to a thread-local static variable.
+
+Erroneous code example:
+
+```compile_fail,E0625
+#![feature(thread_local)]
+
+#[thread_local]
+static X: usize = 12;
+
+const Y: usize = 2 * X;
+```
+
+Static and const variables can refer to other const variables but a const
+variable cannot refer to a thread-local static variable. In this example,
+`Y` cannot refer to `X`. To fix this, the value can be extracted as a const
+and then used:
+
+```
+#![feature(thread_local)]
+
+const C: usize = 12;
+
+#[thread_local]
+static X: usize = C;
+
+const Y: usize = 2 * C;
+```