cmdline-opts: expand the parallel explanations

Closes #13886

docs/cmdline-opts/parallel.md

@@ -11,11 +11,22 @@ Scope: global
 See-also:
   - next
   - verbose
+  - parallel-max
+  - parallel-immediate
 Example:
   - --parallel $URL -o file1 $URL -o file2
 ---
 
 # `--parallel`
 
-Makes curl perform its transfers in parallel as compared to the regular serial
-manner.
+Makes curl perform all transfers in parallel as compared to the regular serial
+manner. Parallel transfer means that curl runs up to N concurrent transfers
+simultaneously and if there are more than N transfers to handle, it starts new
+ones when earlier transfers finish.
+
+With parallel transfers, the progress meter output is different than when
+doing serial transfers, as it then displays the transfer status for multiple
+transfers in a single line.
+
+The maximum amount of concurrent transfers is set with --parallel-max and it
+defaults to 50.