[spec/type] Improve numeric type conversion examples

Move integer promotion examples to that section and show dchar promotion.
Add rationale for integer promotion.
Add rationale for numeric type conversion rules.
Rename int, uint variables for signed/unsigned example and show
long/ulong.
Add floating point example.

Author: ntrel

spec/type.dd

@@ -274,6 +274,28 @@ $(H3 $(LEGACY_LNAME2 Integer Promotions, integer-promotions, Integer Promotions)
     column.
     )
 
+    $(SPEC_RUNNABLE_EXAMPLE_COMPILE
+    ---
+    byte a;
+    auto b = a + a;
+    static assert(is(typeof(b) == int));
+    // error: can't implicitly convert expression of type int to byte:
+    //byte c = a + a;
+
+    ushort d;
+    // error: can't implicitly convert expression of type int to ushort:
+    //d = d * d;
+    int e = d * d; // OK
+    static assert(is(typeof(int() * d) == int));
+
+    dchar f;
+    static assert(is(typeof(f - f) == uint));
+    ---
+    )
+
+    $(RATIONALE 32-bit integer operations are often faster than smaller integers
+    for single variables on modern architectures.)
+
 $(H3 $(LEGACY_LNAME2 Usual Arithmetic Conversions, usual-arithmetic-conversions, Usual Arithmetic Conversions))
 
     $(P The usual arithmetic conversions convert operands of binary
@@ -310,21 +332,30 @@ $(H3 $(LEGACY_LNAME2 Usual Arithmetic Conversions, usual-arithmetic-conversions,
     )
     )
 
+    $(RATIONALE The above rules follow C99, which makes porting code from C easier.)
+
+    $(P $(B Example:) Signed and unsigned conversions:)
     $(SPEC_RUNNABLE_EXAMPLE_COMPILE
     ---
-    byte a;
-    int b = a * a;
-    auto c = a + a;
-    static assert(is(typeof(c) == int));
+    int i;
+    uint u;
+    static assert(is(typeof(i + u) == uint));
+    static assert(is(typeof(short() + u) == uint));
+    static assert(is(typeof(ulong() + i) == ulong));
+    static assert(is(typeof(long() - u) == long));
+    static assert(is(typeof(long() * ulong()) == ulong));
+    ---
+    )
 
-    ushort d;
-    static assert(is(typeof(b * d) == int));
+    $(P $(B Example:) Floating point:)
+    $(SPEC_RUNNABLE_EXAMPLE_COMPILE
+    ---
+    float f;
+    static assert(is(typeof(f + ulong()) == float));
 
-    uint e;
-    static assert(is(typeof(b + e) == uint));
-    static assert(is(typeof(d + e) == uint));
-    static assert(is(typeof(ulong() + b) == ulong));
-    static assert(is(typeof(long() - e) == long));
+    double d;
+    static assert(is(typeof(f * d) == double));
+    static assert(is(typeof(real() / d) == real));
     ---
     )