doc: align Buffer.concat documentation with behavior

Author: gurgunday

doc/api/buffer.md

@@ -1039,7 +1039,7 @@ If the list has no items, or if the `totalLength` is 0, then a new zero-length
 If `totalLength` is not provided, it is calculated from the `Buffer` instances
 in `list` by adding their lengths.
 
-If `totalLength` is provided, it is coerced to an unsigned integer. If the
+If `totalLength` is provided, it must be an unsigned integer. If the
 combined length of the `Buffer`s in `list` exceeds `totalLength`, the result is
 truncated to `totalLength`. If the combined length of the `Buffer`s in `list` is
 less than `totalLength`, the remaining space is filled with zeros.

test/parallel/test-buffer-concat.js

@@ -72,6 +72,14 @@ assert.throws(() => {
            'or Uint8Array. Received type number (3)'
 });
 
+assert.throws(() => {
+  Buffer.concat([Buffer.from('hello')], 3.5);
+}, {
+  code: 'ERR_OUT_OF_RANGE',
+  message: 'The value of "length" is out of range. It must be an integer. ' +
+           'Received 3.5'
+});
+
 // eslint-disable-next-line node-core/crypto-check
 const random10 = common.hasCrypto ?
   require('crypto').randomBytes(10) :