Add cross reference

Nothing to add about inferring the error type thrown by a function,
because that behavior applies only to closures and do-catch blocks.

Author: amartini51

TSPL.docc/ReferenceManual/Declarations.md

@@ -1476,16 +1476,14 @@ func <#function name#>(<#parameters#>) throws(<#error type#>) -> <#return type#>
 Calls to a throwing function or method must be wrapped in a `try` or `try!` expression
 (that is, in the scope of a `try` or `try!` operator).
 
-<!-- XXX
-Rule for inferring the thrown error type
-Xref to the guide <doc:ErrorHandling#Specifying-a-Concrete-Error-Type>
--->
-
 A function's type includes whether it can throw an error,
 and what type of error it throws.
 This means, for example, you can use a nonthrowing function
 in a context where a throwing one is expected.
-For more information, see <doc:Types#Function-Type>.
+For more information about the type of a throwing function,
+see <doc:Types#Function-Type>.
+For examples of working with errors that have explicit types,
+see <doc:ErrorHandling#Specifying-a-Concrete-Error-Type>.
 
 You can't overload a function based only on whether the function can throw an error.
 That said,

TSPL.docc/ReferenceManual/Types.md

@@ -351,6 +351,7 @@ that can throw or rethrow an error must include the `throws` keyword.
 <!-- XXX TR: Confirm rethrowing functions use 'throws' -->
 You can include a type after `throws` in parentheses
 to specify the type of error that the function throws.
+The throw error type must conform to the `Error` protocol.
 Writing `throws` without specifying an type
 is the same as writing `throws(any Error)`.
 Omitting `throws` is the same as writing `throws(Never)`.