[spec/expression] Improve assert(0) docs

Add example.
Add rationale for halt.

Author: ntrel

spec/expression.dd

@@ -2744,11 +2744,30 @@ $(H4 $(LNAME2 assert-ct, Compile-time Evaluation))
     Compile Time Function Execution (CTFE) is not attempted.
     )
 
+    $(P This allows the compiler to suppress an error when there is a
+    missing return statement:)
+
+    $(SPEC_RUNNABLE_EXAMPLE_COMPILE
+    ---
+    int f(int x)
+    {
+        if (x > 0)
+        {
+            return 5 / x;
+        }
+        assert(0);
+        // no need to use a dummy return statement here
+    }
+    ---
+    )
+
     $(P The implementation may handle the case of the first $(I AssignExpression) evaluating to `false`
     at compile time differently - even when other `assert`s are ignored,
     it may still generate a $(D HLT) instruction or equivalent.
     )
 
+    $(RATIONALE Halting the program prevents undefined behaviour from occurring.)
+
     $(P See also: $(DDSUBLINK spec/version, static-assert, `static assert`).)
 
 $(H4 $(LNAME2 assert-message, Assert Message))
@@ -2765,7 +2784,7 @@ $(H4 $(LNAME2 assert-message, Assert Message))
         }
         ----
 
-    $(P When compiled and run, it will produce the message:)
+    $(P When compiled and run, typically it will produce the message:)
 
     $(CONSOLE core.exception.AssertError@test.d(3) an error message)
 

spec/function.dd

@@ -400,7 +400,8 @@ $(H2 $(LNAME2 function-return-values, Function Return Values))
         unless:)
         $(UL
         $(LI the function executes an infinite loop)
-        $(LI the function executes an `assert(0)` statement)
+        $(LI the function executes an $(DDSUBLINK spec/expression, assert-ct,
+            `assert(0)` statement))
         $(LI the function evaluates an expression of type
             $(DDSUBLINK spec/type, noreturn, `noreturn`))
         $(LI the function contains inline assembler code)