Saturate exit codes to 255 for all builtins

After commit 6dd6a57c60, 3 remaining
builtins were affected by uint8_t overflow: `exit`, `return`, and
`functions --query`.

This commit:
- Moves the overflow check from `builtin_set_query` to `builtin_run`.
- Removes a conflicting int -> uint8_t conversion in `builtin_return`.
- Adds tests for the 3 remaining affected builtins.
- Simplifies the wording for the documentation for `set --query`.
- Does not change documentation for `functions --query`, because it does
  not state the exit code in its API.
- Updates the CHANGELOG to reflect the change to all builtins.

src/builtin_set.cpp

@@ -565,13 +565,6 @@ static int builtin_set_query(const wchar_t *cmd, set_cmd_opts_t &opts, int argc,
         free(dest);
     }
 
-    // The return value is cast to an 8-bit unsigned integer,
-    // so saturate the error count to 255.
-    // Otherwise 256 errors is reported as 0 errors.
-    if (retval > 255) {
-        retval = 255;
-    }
-
     return retval;
 }