Point from FsComp.txt to error numbers before 200 (#15875)

src/Compiler/FSComp.txt

 # -------------------------------------------------------------------------------
 # use a completely new error number and keep messages in their surrounding groups
 # -------------------------------------------------------------------------------
+# Error numbers before 200 are listed in src/Compiler/Driver/CompilerDiagnostics.fs from the line with "member exn.DiagnosticNumber ="
 undefinedNameNamespace,"The namespace '%s' is not defined."
 undefinedNameNamespaceOrModule,"The namespace or module '%s' is not defined."
 undefinedNameFieldConstructorOrMember,"The field, constructor or member '%s' is not defined."
@@ -1723,4 +1724,4 @@ featureAccessorFunctionShorthand,"underscore dot shorthand for accessor only fun
 featureUnmanagedConstraintCsharpInterop,"Interop between C#'s and F#'s unmanaged generic constraint (emit additional modreq)"
 3578,chkCopyUpdateSyntaxInAnonRecords,"This expression is an anonymous record, use {{|...|}} instead of {{...}}."
 3579,alwaysUseTypedStringInterpolation,"Interpolated string contains untyped identifiers. Adding typed format specifiers is recommended."
-3580,tcUnexpectedFunTypeInUnionCaseField,"Unexpected function type in union case field definition. If you intend the field to be a function, consider wrapping the function signature with parens, e.g. | Case of a -> b into | Case of (a -> b)."
\ No newline at end of file
+3580,tcUnexpectedFunTypeInUnionCaseField,"Unexpected function type in union case field definition. If you intend the field to be a function, consider wrapping the function signature with parens, e.g. | Case of a -> b into | Case of (a -> b)."