Added documentation on use of regex

Here you can use regex to refer to argument in a list
For example in
t1: TORSION ATOMS=5,7,9,15
t2: TORSION ATOMS=7,9,15,17
d1: DISTANCE ATOMS=7,17 COMPONENTS
PRINT ARG=(d1.[xy]|t[0-9]) STRIDE=100 FILE=colvar FMT=%8.4f