Add rules on extern "custom" fn() signatures

On the lang side, we decided the signature rules for `extern "custom"`
function pointers should match those for `extern "custom"` function
items.  Let's consolidate the signature rules on function items into a
single rule then cite that rule normatively from the function pointer
types chapter.

Author: traviscross

src/items/functions.md

@@ -244,14 +244,12 @@ For other considerations and limitations regarding unwinding across FFI boundari
 r[items.fn.extern.custom.intro]
 An `extern "custom"` function has an unknown, custom ABI. The only way to call such a function is via [inline assembly].
 
-r[items.fn.extern.custom.safety]
-An `extern "custom"` function must be `unsafe`.
+r[items.fn.extern.custom.signature]
+The signature of an `extern "custom"` function must:
 
-r[items.fn.extern.custom.parameters]
-An `extern "custom"` function must not have any parameters.
-
-r[items.fn.extern.custom.return-type]
-An `extern "custom"` function must return the [unit type].
+- Be `unsafe`.
+- Not have any parameters.
+- Return the [unit type].
 
 r[items.fn.extern.custom.naked]
 An `extern "custom"` function definition must be a [naked function].

src/types/function-pointer.md

@@ -50,6 +50,9 @@ The `unsafe` qualifier indicates that the type's value is an [unsafe function],
 r[type.fn-pointer.constraint-variadic]
 For the function to be variadic, its `extern` ABI must be one of those listed in [items.extern.variadic.conventions].
 
+r[type.fn-pointer.extern-custom]
+The signature of an `extern "custom"` function pointer must follow the rules in [items.fn.extern.custom.signature].
+
 r[type.fn-pointer.attributes]
 ## Attributes on function pointer parameters