Change help to make current -n/-k behavior a Feature

[davidBar-On]

• Version of iperf3 (or development branch, such as master or
  3.1-STABLE) to which this pull request applies:
  master

• Issues fixed (if any): workaround for bytes/blockcount with reverse mode #596, Lower throughtput numbers reported with v3.16 #1768

• Brief description of code changes (suitable for use as a commit message):

Currently, there are two issues with the --bytes and --blockcount options:

• Test ends only at the end of the interval when at least the specified number of bytes/blocks were sent ( Issue Lower throughtput numbers reported with v3.16 #1768 - issue started with the multi-thread versions, and PR End test immediately when all number of blocks/bytes sent/received #1775 is a suggested fix).
• The bytes/blocks count is done only by the client, even in reverse mode (Issue bytes/blockcount with reverse mode #596).

The purpose of this PR to make the current behavior a feature by documenting it in the help message, assuming that these issues are not going to be fixed.

[swlars]

Thanks for the pull request! This looks like a pretty straightforward request, so we should be getting to it soon.

[bmah888]

This looks good, thanks! Merging shortly.