Clarified use of table-extension..

Following some discussion on gmx-users of the implications of
table-extension, added a clarifying remark in mdp_opt.html for
online and printed manual.  I did not add all of the suggested
fixes, as most were redundant, and this single sentence should
alleviate anything that may have been unclear.

Change-Id: I89bcecee2d0b11149423990f957b7092b0f083b9