os/shell: do_os_system(): Always show last chunk.

This ameliorates use-cases like:
    :!cat foo.txt
    :make
where the user is interested in the last few lines of output.

Try these shell-based ex-commands before/after this commit:
    :grep -r '' *
    :make
    :!yes
    :!grep -r '' *
    :!git grep ''
    :!cat foo
    :!echo foo
    :!while true; do date; done
    :!for i in `seq 1 20000`; do echo XXXXXXXXXX $i; done

In all cases the last few lines of the command should always be shown,
regardless of where throttling was triggered.

1 files changed, 3 insertions, 4 deletions

diff --git a/runtime/doc/vim_diff.txt b/runtime/doc/vim_diff.txt
index 1940bc55fa..7ccdfd2bdd 100644
--- a/runtime/doc/vim_diff.txt
+++ b/runtime/doc/vim_diff.txt
@@ -155,10 +155,9 @@ are always available and may be used simultaneously in separate plugins.  The
 
 |system()| does not support writing/reading "backgrounded" commands. |E5677|
 
-Nvim truncates ("throttles") shell-command messages echoed by |:!|, |:grep|,
-and |:make|. No data is lost, this only affects display. This makes things
-faster, but may seem weird for commands like ":!cat foo". Use ":te cat foo"
-instead, |:terminal| output is never throttled.
+Nvim may throttle (skip) messages from shell commands (|:!|, |:grep|, |:make|)
+if there is too much output. No data is lost, this only affects display and
+makes things faster. |:terminal| output is never throttled.
 
 |mkdir()| behaviour changed:
 1. Assuming /tmp/foo does not exist and /tmp can be written to