A few more changes to be consistent and clear.
* q is for quit (mnemonics)
* tab completion seemed to have invalid examples/instructions
* made a table of less commands
* explained why less commands work in man
* redirection exercise was ambiguous: files that contain 4 in their contents, or whose names contain the number 4?
* mkdir is to 'make' a directory (mnemonics)
* combined sort exercise block