Update comments for {f16, f32, f64, f128}::midpoint

Clarify what makes some operations not safe, and correct comment in the
default branch ("not safe" -> "safe").

Author: tgross35

Diff for: core/src/num/f128.rs

@@ -851,13 +851,13 @@ impl f128 {
             // Overflow is impossible
             (a + b) / 2.
         } else if abs_a < LO {
-            // Not safe to halve a
+            // Not safe to halve `a` (would underflow)
             a + (b / 2.)
         } else if abs_b < LO {
-            // Not safe to halve b
+            // Not safe to halve `b` (would underflow)
             (a / 2.) + b
         } else {
-            // Not safe to halve a and b
+            // Safe to halve `a` and `b`
             (a / 2.) + (b / 2.)
         }
     }

Diff for: core/src/num/f16.rs

@@ -880,13 +880,13 @@ impl f16 {
             // Overflow is impossible
             (a + b) / 2.
         } else if abs_a < LO {
-            // Not safe to halve a
+            // Not safe to halve `a` (would underflow)
             a + (b / 2.)
         } else if abs_b < LO {
-            // Not safe to halve b
+            // Not safe to halve `b` (would underflow)
             (a / 2.) + b
         } else {
-            // Not safe to halve a and b
+            // Safe to halve `a` and `b`
             (a / 2.) + (b / 2.)
         }
     }

Diff for: core/src/num/f32.rs

@@ -1070,13 +1070,13 @@ impl f32 {
                     // Overflow is impossible
                     (a + b) / 2.
                 } else if abs_a < LO {
-                    // Not safe to halve a
+                    // Not safe to halve `a` (would underflow)
                     a + (b / 2.)
                 } else if abs_b < LO {
-                    // Not safe to halve b
+                    // Not safe to halve `b` (would underflow)
                     (a / 2.) + b
                 } else {
-                    // Not safe to halve a and b
+                    // Safe to halve `a` and `b`
                     (a / 2.) + (b / 2.)
                 }
             }

Diff for: core/src/num/f64.rs

@@ -1064,13 +1064,13 @@ impl f64 {
             // Overflow is impossible
             (a + b) / 2.
         } else if abs_a < LO {
-            // Not safe to halve a
+            // Not safe to halve `a` (would underflow)
             a + (b / 2.)
         } else if abs_b < LO {
-            // Not safe to halve b
+            // Not safe to halve `b` (would underflow)
             (a / 2.) + b
         } else {
-            // Not safe to halve a and b
+            // Safe to halve `a` and `b`
             (a / 2.) + (b / 2.)
         }
     }