add long error explanation for E0625

compiler/rustc_error_codes/src/error_codes/E0625.md

@@ -0,0 +1,28 @@
+Static and const variables can refer to other const variables. But a const
+variable cannot refer 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;
+```
+
+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;
+```
+