Add clarification to wfUseMW() phpdoc

Add clarification to wfUseMW() phpdoc

While reviewing Ia5a7661c I happened to stumble in to the dark labyrinth
of PHP's version_compare() function. This comment attempts to leave
bread crumbs behind for the next unwary adventurer who stumbles into the
same damp cave.

The tricky bit is that when '1.22wmf22' is compared with '1.22'
internally it is as though you are comparing '1.22.wmf.22' with
'1.22.0.0'. By the component-wise comparison then performed 'wmf' is
considered the lowest priority value that could be in the third
position. When using '1.22c' for the required version the 'c' is given
the same lowest priority as 'wmf'.

Change-Id: I58aeb8dc6b1ff31897d72eb44904d96487ad3653