Clarify the representation of icmp output (#6202)

* Clarify the representation of `icmp` output

* Reformat

* "ie" => "i.e."

* Update `fcmp` documentation as well

cranelift/codegen/meta/src/shared/instructions.rs

@@ -1615,6 +1615,14 @@ pub(crate) fn define(
 
         When this instruction compares integer vectors, it returns a vector of
         lane-wise comparisons.
+
+        When comparing scalars, the result is:
+            - `1` if the condition holds.
+            - `0` if the condition does not hold.
+
+        When comparing vectors, the result is:
+            - `-1` (i.e. all ones) in each lane where the condition holds.
+            - `0` in each lane where the condition does not hold.
         "#,
             &formats.int_compare,
         )
@@ -2741,6 +2749,14 @@ pub(crate) fn define(
 
         When this instruction compares floating point vectors, it returns a
         vector with the results of lane-wise comparisons.
+
+        When comparing scalars, the result is:
+            - `1` if the condition holds.
+            - `0` if the condition does not hold.
+
+        When comparing vectors, the result is:
+            - `-1` (i.e. all ones) in each lane where the condition holds.
+            - `0` in each lane where the condition does not hold.
         "#,
             &formats.float_compare,
         )