Clarify actions of head and tail

Author: bkmgit

episodes/04-pipefilter.md

@@ -746,8 +746,8 @@ and *only* the processed data files?
 - `wc` counts lines, words, and characters in its inputs.
 - `cat` displays the contents of its inputs.
 - `sort` sorts its inputs.
-- `head` displays the first 10 lines of its input.
-- `tail` displays the last 10 lines of its input.
+- `head` displays the first 10 lines of its input by default without additional arguments.
+- `tail` displays the last 10 lines of its input by default without additional arguments.
 - `command > [file]` redirects a command's output to a file (overwriting any existing content).
 - `command >> [file]` appends a command's output to a file.
 - `[first] | [second]` is a pipeline: the output of the first command is used as the input to the second.