link to wherever REGEXP info is kept

[Dan Jacobson]

On the nl info page
    `pREGEXP'
          number only lines that contain a match for REGEXP.

there should be a link at this point to more about REGEXPs,
(even though we are supposed to know how to get to wherever that info
is put on our own or not.)

Furthermore, your nl command appears to be smoking different stuff
than your grep command:

$ echo b|grep '[[:alnum:]]'
b
$ echo -e a\\nb|nl -bp'[:alnum:]'
     1  a
       b
$ echo -e a\\nb|nl -bp'[[:alnum:]]'
       a
       b

This makes it even more imperative that the nl info and man pages
define just what types of REGEXPs they mean.