Change diagnostic message to point at docs.

Author: jonmeow

toolchain/diagnostics/diagnostic.h

@@ -119,9 +119,9 @@ struct DiagnosticBase {
       : Kind(kind), Level(level), Format(format) {
     static_assert((... && !(std::is_same_v<Args, llvm::StringRef> ||
                             std::is_same_v<Args, llvm::StringLiteral>)),
-                  "For diagnostics, use a format provider (see "
-                  "toolchain/diagnostics/format_providers.h) or std::string to "
-                  "avoid lifetime issues.");
+                  "String type disallowed in diagnostics. See "
+                  "https://github.com/carbon-language/carbon-lang/blob/trunk/"
+                  "toolchain/docs/diagnostics.md#diagnostic-parameter-types");
   }
 
   // The diagnostic's kind.

toolchain/docs/diagnostics.md

@@ -167,9 +167,11 @@ methods for formatting arguments:
     -   This includes `char` and integer types (`int`, `int32_t`, and so on).
     -   String types can be added as needed, but stringifying values using the
         methods noted below is preferred.
-        -   Use `llvm::StringLiteral` where appropriate; use `std::string` when
-            allocations are required.
+        -   Use `std::string` when allocations are required.
         -   `llvm::StringRef` is disallowed due to lifetime issues.
+        -   `llvm::StringLiteral` is disallowed because format providers such as
+            `BoolAsSelect` should work in cases where a `StringLiteral` could be
+            used.
 -   `llvm::format_provider<...>` specializations.
     -   `BoolAsSelect` and `IntAsSelect` from
         [format_providers.h](/toolchain/diagnostics/format_providers.h) are