mention grep on directories will never find anything

[jidanni]

       -d ACTION, --directories=ACTION
              If  an input file is a directory, use ACTION to process it.  By 
default, ACTION is read, which means that
              directories are read just as if they were ordinary
              files.

At this point perhaps mention that
$ grep . /
$
nothing will ever be found, pcregrep too.

And mention why you don't say like
$ cat /
cat: /: Is a directory
(Probably POSIX or something.)
grep (GNU grep) 2.5.1